[Unreal]

From your question, I assume the motor is long gone. Unfortunately, the 68 trim tag tells very little about the car's pedigree. Without original motor, or paperwork the best you can do is work backwards through previous owners ---made easier by the fact that your friend is one, so he can start backwards from the mid 90's.

If you can already establish that it was a b/b, then you can look for remains of which one. Most notably, the L78 had a single fuel line, so check that, as well as the fuel line clips and sending unit.

Good luck with the research. To me, that's part of the fun of the hobby.