Heater core substitute

[mwredit]

I'm wondering if anyone has replaced their heater core with another that is close to the original size?  I'm looking for a common replacement that will fit close to the original.  The closest that I've found measures 8 1/4" x 6" x 2" (application for 1970-1981 Chevy Camaro-Pontiac Firebird), which is close to the original size (10 3/4" x 5 7/8" x 2").  The inlet and outlets seem to match the original locations, except where the heater control valve attaches, which lacks the two stud flanged attachment.  I dont want to modify the heater box in any way, except internally if need be.  Has anyone had experiences with this?  Thoughts?

Thanks

[squiglyzigly]

Are you having trouble locating a s/h replacement or is this not the route you want to go?

[mwredit]

Are the originals fairly easy to get?  That's my only real concern.