I have given a bad bit of swap advice.
I have mentioned in the past, that a blower motor from a Caddy would swap right into our trucks and blow harder.
Today while at the junk yard, I grabbed a Caddy heater(late 70's eldo) and went to install it in a 71 truck, and it was a no-go. It would not fit.
I have read this over and over in magizines, and spoken to a few who have done it. Possably it would have to be an older Caddy that the part needs to come off of.
I'll try to look into it some more. I hope no one has lost any $$ becouse of my bad tip.