If you're building a 420 stroker, why not use a 4 bolt main block for a little extra insurance IMO. You got the right idea with the dual TBI's becuase that is the only way you will be able to supply enough fuel to feed the motor. I'm guessing your 420 should make over 400 HP when all is said and done.
Get a 255 LPH high volume fuel pump and you should be O.K with that. I think it supports something like 450 or 500 HP.
You're right though, tuning this thing is probably going to be a real biotch.