I've settled on a Chevy 235 out of an 1958-9 3700 truck
The engine SN is FIII9K - Flint, MI Nov. 19 34-35-3700 truck
Head - 3836848
Block - 3764476 58-62 235
K 18 8 December 18th, 1958
Its a higher compression motor (8.5:1) and it was a truck motor... so hopefully its going to have enough get up and go to handle these damn Appalachian mountains I live in. But it is a full pressure motor... the only thing I was hoping for was for hydraulic lifters... from what I understand these are still mechanical... that means setting tappet gaps
The only real difference was in 58 the motor mounts for trucks moved from under the crank pulley to the sides of the engine and the water pump has a longer/deeper pulley. The flange on the 58's oil chain housing can be drilled for mount bolts (going to use the G506's original front motor mount pad) and there are options to fix the water pump... either an adapter plate or simply shorten the shaft on the 58 water pump.
The G506's tranny mounted right up to the bellhousing and the rear motor mounts seem to fit into the frame mounts. Tomorrow I will test the engine on the stand and then sandblast/paint if it runs.