Did same to mine and works really well heater cores seem to be obsolete unless you want to pay a lot to have one built. I used one from a 1960 c10 which is really close to original routing and had to add supports off old one where core mounts to housing. Also drive motor was changed using one from 1979 mustang. as all these parts were new and still out there. Very pleased with results.

1957 Chevy 3100
"Finally back on the road"
Follow in the DITY