I've searched the threads and understand the pressure side of the system on a 350/383 is from the intake and the return would be to the pump.

So I need to plumb from the intake manifold to the inlet port on the heater.

But....

Which hose barb on the heater is the inlet side, the 3/4" or the 5/8"...
Sorry for such a lame question :dontknow:

5/8 to the manifold, 3/4 to the pump.

Should look like this (hope it's clear enough)

http://i564.photobucket.com/albums/ss83/rushgator/cars/img_0309-1.jpg