Ford Heater Core in a Chevy?

[jasonrog]

My heater core started to leak in my 71 1/2 ton. I does not have air conditioning. The PO had installed a core from a ford about 15 years ago. This was done because it was slightly bigger in dimension than the stock core. I must now replace it and do not have a clue on how to source one out. Has anyone else put a larger core in and what application is it for? I was told that the heater box was not modified so I can use the stock one if I can't figure out what is in there now. I really like the idea of a bigger core to blow more heat.

Thanks
Jason