70 Dart heater box questions

Jumping in to my 69 Fish Tomorrow to replace the heater core! It has lasted since 2014 on a swap meet buy, for rebuild/restoration!

I tossed my original box and all for weight savings in 1986 when drag racing full time. Finished a 7.80 1/8 mile run and the original core let loose. It got hot and steamy on the top end
and just ripped the whole box out between rounds! Put a bypass hose on and forgot about it for near 25 years!