The V3420 was mooted before WW2, but was ultimately suppressed by the need for more V1710s. So WI Allison had given it a bit more attention & gotten approved for, frex, the B-29, instead of the R3350? Or for the A-36, instead of the Merlin? Or P-39 or P-63? How different would the war have been? Would it make any real difference?
One thought: without the R3350s teething problems, the B-29 might have entered service a bit sooner, putting pressure on the Bomb program to deliver a working core sooner...
More V3420s might have impacted civil aviation postwar, with fewer (no?) R3350-powered Connies & such, & no problems with turbocompound turbine failures. (Can that be traced to the cause of any fatal wrecks?)