Dave is right. Also don't over look inline 6's man! I want to build a inline 6 right now pretty darn bad because I think that the 235's and up are nice for a street rod and you can make some good power and they have excellent torque. They might not pull out any 350-400 HP like a little sbc will but I do know you can easily make 200 horse power from them (Auto Restorer has all this info on a 235 build and the guy made 200 HP with a little exhaust work, porting and intake/carb work. I don't like Ford or nothing (mostly because I grew up with my dad liking Chevy's) but I would love to build a flat head and then now I wish that I could have had a 401 nailhead since Dave gave me some info on them and I never knew they were good motors. There's a lot of different things out there and you can pick up something like a inline 6 for pretty cheap these days and they are cheap to build and still good on gas.
Well I don't wan't to lecture you or nothing, I just wanted to give you my oppinions. Good luck on whatever you choose.