Check out the Hurricane made by Old Air Products. The unit is about the same size as the stock heater. I am intending to use this but I am still at least a year away from buying anything. They are located south of Fort Worth Texas.

Somewhere I read some complaints about the performance but most likely it had to do with poor insulation of the cab and air leaks.

I hope this helps. Its the only unit I have seen that looks like it would fit without removing out the cowl vent.

1946 Chevy Half Ton
('56 235, '86 T5, '79 Camaro Z28 rear end, working on the '85 Jaguar XJ6 IFS)
1976 Margay Panther X, MC91B