Yep, cleaner the better. Besides it will hold the new paint better if it's clean. You could beatblast, just make sure you get all the media out after.
No need to change cams(unless you want to) just for a 4bbl swap. Cliff, the carb shop. If you're going to rebuild it you might want to get Cliff's book on the subject.

Don't expect miracles from just bolting on a 4bbl though. Those stock 2bbl dump alot of gas too. You might not get any more performance out of it except it'll sound better when the secondaries open up. That'll make you smile.