Pretty much everything must be stock and remain in place except the back seat, which may be removed if a roll cage is installed. The back seat area would then have to be replaced with a firewall and covered in some sort of upholstery. You can add aftermarket gauges, a shifter, and a steering wheel.
You need to go here:
http://www.nhra.com/competition/classification.aspx
That will let you know which engine/transmission combinations are legal in your car. Just scroll down to the "Chevrolet" section and click on "1965". You'll find everything from a 153 4 cylinder at 90HP, to a 327/300HP V8 at 265HP.
Then go to:
http://www.nhra.com/competition/blueprints.aspx
That gets you the engine blueprint specifications for the engine you want to use. Again, scroll down to the "Chevrolet" section and click on "1965". You'll find all the necessary specifications in the engines there, including
casting numbers for heads and intakes.
You'll need Excel, or Excel reader to read the classification guide, and Word to read the blueprint guide.
At the top of those pages, there is a menu that you can scroll over that is entitled "Competition", there you'll find legal replacement pistons, rods, and other parts. You also need a printed copy of the rule book.
Welcome to the asylum, and good luck.