Any reason why I can't use a factory non air conditioning heater box instead of the 'AC delete' box sold by vendors for $120 more. Taking AC out of 79 Z28. Want to keep heat as it'll be a daily driver. Thanks for any input

 

Check the firewall hole between the two models.
Most A/C equipped have a vertical rectangle hole, maybe 4 X 10 inches?
Non-A/C have a horizontal hole, maybe 4 X 6 inches?


Might look here to see the difference.
Body Components - Firewall - Firewalls


If you got another box, you could it and make a cover for it in retaining the same blower.