I think as long as the seat vbelts use the same mounting points, you're right on that.
I have a black headlining, possible original but not too sure.
If you can't see the headlining when driving normally, don't get too stressed about it,
just make it the same colour as the seats or carpet.

I asked the same question about 12 months ago (in regards to the seatbelts). The general consensus at that time was that its illegal to do so in most states. Thats why most seatblets are/were cut after use in a vehicle. I did it anyway so if it fits well and operates 100%... i say go for it. Also make sure noe tears or rips in the seatbelt sash...

Just to clarify, it is (supposedly) illegal to re-use any safety device from a car
(eg, seatbelts), but you can use new ones.