Frank, the cast aluminum mounting feet you have are designed to be used with cast aluminum mounting plates like the ones here
at Rex Marine, for example. They were not intended to be used with flate aluminum plate-type fabricated motor plates.
With the cast motor plates, the surface the mounting feet bolt to is 1/2" BEHIND the bellhousing face and 1/2" AHEAD of the front face of the block. That means a BBC will have a mounting foot spacing of approx 24.6"(23.6" for motor and 1/2" at each end) from front of front foot to front of rear foot.
By the dimensions you gave for those feet, I suspect the previous engine was an Oldsmobile. One positive is that the Olds and Chevy should be the same distance from the pump IF they both were/are used with flywheel or both were/are used with flexplate. Most Olds seemed to use a flywheel. If your BBC uses a flexplate, you should be OK, but don't let the driveline run too far from full engagement on pump shaft. The best approach here is to get the correct mounts for the BBC, as well as the flexplate/flywheel you plan to use and the appropriate PTO and bolt everything together for a test fit. MAKE SURE THE DRIVELINE DOES NOT BOTTOM OUT ON THE PUMP SHAFT! A 1/4" or so from bottomed out is about right.
EDIT: Just pulled out some notes on a previous boat and found that the Olds mount spacing was ~25.25" so Im not sure what those mounts would have fit. Notch for starter is also on the wrong side for an Olds. Maybe it was a Ford 460? Still start by bolting up the parts you plan to use and then bolt the bellhousing to the rear feet and see where that leaves you.
