Trans-Am info for the 1966 Barracuda
Jari, Thanks for posting the link. I have an article about Juha written in Finnish I think. I couldn't find anyone to translate!
In the US it is not so simple. SCCA is usually acknowledged as the setting the rules standard for amatuer autocross, rally, and roadracing. But they have little interest in vintage and historic competition so sanctioning is like the many statesand provinces - each has its own rules.
Most of the vintage groups are not limited to original race cars. SVRA is one of the groups that is restricted to original racecars but they do make an exemption for under represented cars. The 64-66 Darts are now given that allowance, thanks to Dave at Lame Horse Racing.
SCCA does have a few places to play for a older sedans/saloons - some regions have a vintage class and some owner/drivers choose to run in the newish SPO class. That has many allowances as its a relatively open class.
All this means is a lot of decisions early on about which set of rules to build to.