where to start...
the engine you are getting just had bearings put in it...
align honed or bored? plasti-gaged what clearance?
downshifting because of lack of power or maladjusted transmission?
what RPM is this downshift happening?
what gear ratio/tire height in the rear?
stock injector system?
if using a carbuetor the sizing depends on size and rpm level.
a 355 @ 6000rpm needs a minimum of 650cfm @ WOT.
see my signature for gains and exhaust....