To convert from a 2bbl Cleveland to a 4bbl Cleveland basically you need the heads of course, 4bbl intake and 4bbl manifolds/headers. Its a simple swap if your rebuilding the engine anyway. IM doing this right now, I bought 1970 4bbl quench heads for my 1972 2bbl engine. Its only costing me 250 dollars more, the price I paid for the heads to have it done. Either way my 2bbl heads would have needed rebuilt so I figured mines well go for some real power. Plus I wanted a new intake, headers and cam anyway for the 2bbl rebuild. Its amazing the HP I see comming from 4v Clevelands with nothing more then a new carb, intake, headers, and cam. The quench heads on the Cleveland were rated at 300HP stock so you can imagine what nice headers, intake, carb and cam will do for it.
Jim