If you have a correct 40 complete engine, I would at least inspect that and see what condition it's in. The babbit bearings on the rods were installed with shims, so if the crank throws aren't scarred, you can adjust the clearance by removing shims. The main bearings (insert type) also use shims for setting clearance, so could be adjusted as well. That's contingent on the bores and rings being in good shape also. The head from the 1948 motor may not work on the 40 block. 39-40 heads are a different casting number than 41-53 heads. But the 40 head could be reconditioned.
Another option is to try again on the crack repair. You may not have gotten the crack prepped and cleaned properly for your repair to work. Marine-Tex is the recommended epoxy for block crack repairs.
