All 848xxx GTA's have 4-bolts... so this won't help you out in identifying a replica as fake ;-)You see I'd prefer that somebody created a GTA replica out of a 4 bolt front cross member as you will still get the GTA driving experience, but very easily identifiable as a fake ... but I know I am a different planet to you Steve
Dont' forget that this forum contains a "GTA observatory" so any fake GTA that pops-up will soon be noticed
But if you want to pick a starting point for builiding a GTA "look a like" or "replica"... I'm with Steve, value wise it's better to start with a car from the same period (pre sept. 1967).
The chances are greater that it contains all the right bits and pieces for builidng a car that fulfills FIA appendix K for 1965-period racing if you want to take that option (or leave that option open).