The issue you might have is making sure the couplers line up with out any bind. You can bolt the motor and tranny together through the motorplate and install into chassie.Put somthing under the front of the engine off the lower rails and with a screw jack or shims adjust the front of the engine till the couplers are free (not binding). Install some transfer screws into the SBC waterpump holes and either use the BBC mounts (or make your own) to transfer the new bolt pattern to the plate and trim accordingly.
My first post was give because without pics its somtimes very hard to come up with the correct responce. The reason for the crank and upper frame rail being paralle was to be able to slide the front mount back (applies to one peice front mounts) without raising or lowering the front of the motor which in turn would put the driveline couplers out of alignment.
I restored a dragster that was very short coupled , it was rearend to clutch can basicly and although I used a lineup bar to set the engine placement, after it was all said and done, during final assembly with full 392 blown engine weight the frame flexed enough that the couplers bound. I had to pull the lower rear end mounting bolts and rotate (.015) the rear til I could slide the M/F couplers appart and back together and redrill the holes for bigger bolts.
