I reckon carb is too small, ports in the head (if stock) are too small, your cam is unknown and an LCB would be better than the 3 into 1 (if it's just a freeflow).
All these things need addressing if you expect to get real power from it. Just doing some of it won't give a big gain.
I'd start by buying Vizard's book `Tuning BL's A series engine', it will help you see what is needed.
Another good book is Des Hammill's `How to power tune the BMC/BL/Rover 1275 A series engine'.
This last one was on special, at
www.pitstop.net.au last week.
<edit> oil cooler is good for the track, but unless you are doing lots of high speed driving you don't really need it for the road. Just use a good 20W/50, 25W/50 or 20W/60 oil.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.