What thermostat housing are you using? I think the stock 250 thermostat housing layers over to the side. Summit sells a straight thermostat housing for a small block Chevy V8 that should work on your 250. Read the reviews. One guy claims to have used on a 1960 235.

https://www.summitracing.com/parts/sum-g3803


1953 Chevy 3600
1998 Dodge 2500 Cummins 12 valve