Unsure which new Carb to pick

Don't click/buy from the eBay link -- contact the guy (just dug up his email address for you, that is [email protected] ) and have him put together an optimal setup for the Slant-6. That'll be the 1.08" venturi carb with ported vacuum for spark advance, manifold vacuum for PCV and to operate a thermostatic air cleaner, and an integral automatic choke you can and should convert to electric by contacting the folks who sell these.

You will not need to make any other modifications to benefit from this induction upgrade; it'll run well with everything else in stock condition. Of course, it'll run progressively better with other upgrades such as those I linked (ignition, exhaust, etc).

(And one other, other thing: Yes, there is definitely such a thing as too big an exhaust pipe, which will cost you torque. More is not endlessly better; you don't want to create backpressure, but you also want to keep exhaust velocity up, and a giant exhaust pipe kills velocity.)