The numbers stamped on the front of the block follow a standard GM format The first character designates the GM division. The last 8 characters match the last 8 of the vehicle's VIN.
5 (unclear) = GMC division. This does not make sense. We have no records indicating GMC used Buick 350s. Stamping is not clear. If that is a 3 it makes sense where:
3 = Oldsmobile division.
6 = last digit of model year. This must be 1976 based on engine color and year range of when Buick 350s were built.
W = Willow Run, MI. This is the vehicle assembly plant.
128902 = vehicle's individual production sequence number
Attached is a 1976 GM VIN decoder card which shows meaning of each character.
The 430 numbers stamped on the block near the head are a build date code. There should be 2-letters in front of these digits to indicate indicate engine type and usage. We would expect some code between PA and PU if engine was installed in a Buick.
Most of the Buick 350s that year went into Buicks, but some were installed in Pontiac and Oldsmobile X-body cars. The letter codes on the block are probably different for non-Buick cars. The letters and numbers were stamped at different times based on what we have seen on many of these blocks. Sometimes the letters and numbers are stamped so far apart that part of the code is covered by the head gasket. If the heads are going to be removed, the letters may become visible.
Based on the 124 (1?) 748 block casting number and other details, I think there's no doubt it's a Buick 350. If more research is desired we can research other casting numbers on the block, heads, and manifolds. Beyond that, there's the part numbers on the distributor, carb, starter, alternator if the parts look original.