It is very daunting at first to try to find out what is allowed .
But if you go through it bit by bit , & ask a lot of other people , then it can all be done .
https://historicdb.fia.com/sites/def...18_group_1.pdf
That link should get you a copy of the 66 Gp 1 Homologation papers .[ Note , this is a free copy & you should actually have a bought one " non watermarked " to use on events ] .
You also need to read up FIA app K , [ historic regs ]. Which , along with App J of the correct period , will tell you what was allowed for that time .
The Gp 1 Homologation papers only give the original factory specification , but a lot more modifications were allowed under Gp 2 rules of the time .
You also need to know what events you will be doing , because not all countries , series , & individual events run to FIA historic rules , & some have their own variations on them .
That should get you started on some reading up on what to do before you even start on preparing the car .
https://historicdb.fia.com/sites/def...80_group_1.pdf
This should be the Daf 66 ,