1948-1952 truck recirculating heater location?

[FortyNiner]

While you have been clear that others have told you that yours is a 48-52 unit, that just doesn't make sense. The block style Ford came later - like mid 1950s - when the tailgate embossing changed from script style.

You may be trying to fit a 'newer' unit into an older truck and are having the difficulty you've described.