Should I go Big block or Small block?

Big Block definetly! Prefer the 440 however you may consider the 383 just because of the hood clearance issue with the RB blocks. You can solve the header problem a coule of different ways. If you do not want to change out the front end then stick with Dougs for your exhaust. One more thing you might consider the front end swap for the improvement in handling is worth it even if you stay with the traditional small blocks.
I do understand your dillema since I have recently decided to go with a Big Block in my '69 Dart, now I just can't make up my mind between the 440 or the 426. Decisions decisions. Good luck which ever you choose!