55.2-59 Heater assembly diagram.

[dsraven]

LG, can the fan source enough air and also is/are the outlet(s) large enough to allow the fan to discharge fully without restriction?
The heater in my boat called and a new core was not available. I sourced a new core from a common vehicle with inlet/outlets the correct sizes and configuration and then built a new heater around that. I used some of the old heater parts, fan etc, and it works fine again. Just saying if you can't find an original heater then make your own or modify a commonly sized and readily available universal unit for your purpose.