Re: can you replace merc 120 engine block with plain gm 2.5
The 1979 GM (MerCruiser/OMC) has NO resemblence to automotive blocks, of that year range.
As stated, if you need a block, they are readily available at reasonable prices. Just NOT from the auto junkyard.
You will not find an AUTOMOTIVE GM block, of that vinatage to work.
A GM "marine" engine, of that vintage will. Be it MC or OMC.