All very neat, but i would not be relying on nutserts, rivets and self tappers to hold the seat in. For a start it is not safe, and secondly is not legal because the replacement of seats using non-manufacturer mounts needs to be certified which this installation would not pass muster.
Before we go into details, let me say that looking at your photos, i am pretty sure someone with your metal working skills should be able to make up proper reinforcing plate mounted captive nut that is welded under the floor.
Guidance for installation of new seat anchorages can be found here: http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/pdf/NCOP7_Section_LK_Seating_and_Occupant_Protection_1Jan2011_v2.pdf
First of all to applicability, because i know people are going to say, "but it is a pre-ADR vehicle, so i can do what i want" - not true:
VSB-14 wrote: This sub-section applies to all light vehicles and must be read and applied in conjunction with all the LK Codes applicable to the proposed modifications.
Modified vehicles must continue to comply with the Australian Design Rules (ADRs) to which they were originally constructed, except as allowed for in the Australian Vehicle Standards Rules (AVSR). These modified vehicles must also comply with the applicable in-service requirements of the AVSR.
Modified pre-ADR vehicles must continue to comply with the AVSR.
Compliance with the AVSR also means compliance with the equivalent regulations of a State or Territory of Australia.
The reasons why you should use these guidelines are clear, and our circumstances as amateur enthusiasts has been considered:
VSB-14 wrote: Registration Authorities in Australia have recognised the need for guidelines to assist those individuals who wish to construct and install their own additional seats in vehicles. To this end, the following two codes of practice were developed – Vehicle Standards Bulletin VSB 5A, Commercial Manufacture and Installation of Additional Seats and Vehicle Standards Bulletin VSB 5B Construction and Installation of Additional Seats by Individuals.
If these Guidelines are followed closely, the completed installation will be accepted by all State and Territory Registration Authorities as providing an acceptable level of occupant protection without having to demonstrate compliance with the latest edition of the ADRs.
Now to the guts of the problem with your seat mounts:
VSB-14 wrote: 1.1 Seat Anchorage Construction
Single seats may be adequately anchored to sheet steel floors by using not less than four 8mm ISO Grade 8.8 (5/16 inch UNF SAE Grade 5) bolts, with a mild steel reinforcement backing plate of an area not less than 3750mm2 in contact with the mounting surface and a thickness not less than 3mm at each anchorage point. For double and triple seats, the number/size of anchorage points should be increased accordingly.
The shape of the backing plates must match the contour of the floor material. The corners of the reinforcement plates must have a minimum radius of 5mm and the edges adjacent to the floor material must be chamfered.
The anchor bolts must be tightened to the correct tension and fully engage all the thread of the nut.
In cases where seatbelt anchorages are fixed to the seat assembly, the seat anchorages must be certified in accordance with the requirements of Code LK2.
Anchorages must not be fitted through wood, fibreglass, sheet aluminium or plastic or where wood or other non-metal material is sandwiched between steel unless certified in accordance with the requirements of Code LK2.
Unless supported by engineering evidence, seats located over wooden floors must be anchored to the vehicle structure via steel members (e.g. C - section, channel section or rectangular hollow section) of adequate strength to meet the strength requirements specified in VSB 5A or VSB 5B.
Shall we have a look at what VSB-5B says? http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/pdf/vsb_05_b.pdf
The first discrepancy is the sheet channel section you made for the rear, the second is the use of self tappers, nutserts and rivets:
VSB-5B wrote: Seat Construction and Installation
Seat frames must be constructed so that there are no sharp edges or projections which can cause injury to occupants in an impact.
Seat mounts should be located on areas of the structure that are adequately reinforced to take the loads.
Seats from an Existing Vehicle
A seat from another vehicle may be used as an additional seat in another vehicle, eg the bench seat from the front of a sedan may be installed in a cargo van to provide an additional seat.
The seat must be attached to the vehicle with at least the same number and size of fasteners as in the original installation.
Where the seat is installed on a supporting frame to increase the height of the seat above the floor, the supporting frame must be constructed of rectangular hollow section not less than 25 mm x 25 mm x 2 mm. The construction, including bracing, must be as set out in Figures 4, 5 or 6, depending on the number of seating positions provided by the seat.
Now looking at Figure 4, there obviously needs to be some flexibility in the shape of the frame to suit the existing seat and floorpan that you have, but it is pretty clear that flexibility does not extend to the materials or the method of fixing to the floorpan.
So when you say you "fixed the seat mounts", maybe you should think of it as making a template to fix the mounts properly.
cheers
michael
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so I better elaborate a bit on the fitment. In one picture you'll notice the hole in the cross member, the black section with the rivets and the nut fitment . The hole was to allow the fitment of reinforcement plates inside the tunnel, these are pieces of angle which are only held in position by the rivets and actually continue down the front edge of the cross member. The nutset looking inserts aren't normal Sheet metal ones, theyre actually fitted from the inside out. I can tell you, the seat bases would fail long before the mounts will. The seat bases originally have four 6mm bolts holding them to the tracks. I uprated them, plus they only went into frames that are just body metal thickness.