I have a spare bravo III drive, straight back. Has just the one anode under the cavitation plate. It has a crack in the housing. Will the new style lower housing with the twin anodes bolt up to my upper? Or will it only bolt up to the newer style swept back style uppers?

My straight back BIII had both anodes. You can see them in this pic.

Depends on the age. Here is the comparability chart

ok... wow, that's a lot of differences.......

The last change in housing was after 1993 so far as interchangeability

My drive is from 2001. Single anode though.

ok, after 1993 upper and lower did not change so far as newer fitting older. Get a 2023 to 1994 and they should all fit.

Mine was an '04, last year before the swept-back came out.

My 94 27PC came with the 2 piece B3 lower. Picked up a newer 2000 something complete with 1 anode and the lower fit. Later bought a brand new B3 complete (2 anodes) for repower and the gear ratio was to high. Took the new lower off and installed the 2000 something and was still working when I sold the boat this year. Still have the B3 lower

mine is a 2.20 ratio