Just finished a 1970 Challenger. There is a gap between the lower defroster vent housing and the heater box. The last one I did was an A/C car and had a big block of foam to seal the vent to the box. This car is non A/C. Is it a similar setup?

Not the same as a/c. Gap is normal. Do not block or seal.
Jules

The gap allows the warm air flow to mix with passenger compartment air.

Thanks for the info.