If you are going to focus on SBR, then you may as well go all out…
Have the US purchase 2 BOM per turn, and roll the rest tech if your game allows it. Go for Heavy first to maximize damage, then for LRA so that new BOMs can strike immediately. If you are not playing a tech game, then use the remaining income eitehr for a TRN/some land units to reinforce UK assaults from time to time, or save the extra cash and buy 5 BOM every 2 turns.
With that, you can knock Germany down 16 IPC per turn (plus another 3 by UK until their original BOM is lost), and Japan for 3-9 depending on IC builds. US will lose an average of 1 BOM per turn.
From an economic perspective, that leaves UK and USSR facing a Germany with an income in the upper 20’s to low 30’s (equal to UK), and Japan building in the mid to upper 20’s or more, greater than Russia.
And that sounds like a losing proposition to me folks…