You can get replacement cores from your local radiator shop and most of the restoration parts suppliers like Year One and Classic Industries. Your core is the same as '67-'68 Camaro with a small block.
I ordered a core from Classic Industries and it didn't fit. The inlet and outlet tubes were 1/4" closer together than the origional and it wouldn't go thru the holes in the firewall. I discovered this after much struggling to get in inserted from the inside laying upside down.
I took the core out of the case and tried the core only and even that way it wouldn't insert into the holes. I'm working with them now to resolve the issue.
Corvette Store
"67 Firebird heater coil
