I just installed a new heater box in the engine compartment on the firewall and was about to install the new blower motor and fan, but for some reason it won't fit! The blower motor and fan are ~3/8" too deep to seat up against the heater box. Am I missing something here?? This is for my 69 that did not have a heater box at all, just a custom fit cover for the hole in the firewall (which explains why my heater never worked). Thanks!