The years that you are looking at are the first generation of mustangs =)
it depends if you are looking for a GT Shelby 350 or 500. or a Boss 302 or 429
The engine can be a V6 or V8
Horse power can range with the type of mustang that you are looking for.
mustangspecs.com is a good reference to find out what all the options were w/ a mustang for almost any given year.
Also if you really want a 60's mustang.......START SAVING UP NOW!!!
Cause 60's mustang are going for a premium these days (even the junkers) You can expect to spend up to at least 8-10k minimum just finding/buying a car worth restoring....and then expect to put another 15-20k into it for a proper restoration.
Anyway I don't know what you mean by "into my head" but you need to keep researching,reading,and studying if you're at all serious about this. We can only do so much since there is a TON of info you need to know just to be able to buy a decent car (nevermind everything that goes into restoring one too)