Carey's your man if you're not building it yourself. And maybe even after you do build it yourself!
One of our number, David Stroud, did a DIY Sube swap in a pan Based speedster about 7 years ago. As I recall he used a Saab radiator of late '80s vintage, laid almost flat in front of the beam, and ducted air into it on top. He had aluminum spars from a hang glider, 1.5 inches OD, carrying the coolant up and back from the rear. There were a number of bleeders places at the high points to allow trapped air out.
When I swapped a 2.2 into my MG TD replica I modeled my system on his. The MG has a front grill and room for a proper upright rad, so I used one from a Honda Civic. 1.5 aluminized exhaust pipe tucked up tight under the channels along the outer edges of the pan.
Bleeders from a steam/hot water home heating system. . . .
One key factor to understand is that the Suby thermostat works kinda backwards, and it absolutely must have the heater core loop in place and operational in order to work. You can delete the heater core and just loop the fluid back, but you can't omit that loop.
There are several other key factors, most of which I probably don't really understand. @chines1 absolutely does understand them.