If you're going to run without a pump (so using water heat differential to "drive" the water around the loop) IMHO you'd want the radiator "inlet" higher than the "outlet".
Try putting the rad on its side, so one pipe is higher than the other and see if there's any change. If there is, maybe think about something long-ish and single pass, so you can have the inlet 'way higher than the outlet. Like maybe the fedco 2-766 (14.188x3.25x1.625) single pass. Or if you don't want to figure out how to move air through something that odd a shape (I'd just cut a vertical slot in my case to fit and block all other inlets so whatever exhaust fannage you've got will pull air through the rad, the case, then out), then maybe a more square fedco 2-149 (7.75x6.125x2) - also single pass and lots easier to fit - although I'd
guess you'd want the inlet higher than your heat source, so maybe the long, narrow one would actually be easier...
Oh - I have a little experience with using "no pump" cooling systems - tried cooling 2-stroke cylinder heads on a race bike
long ago - worked fine so long as the "rad" (was a car heater core) inlet and outlet were at top and bottom - well enough that we could run leaner (2-strokes are also somewhat gas-evaporation-cooled) which meant fewer pit stops. Airflow through the rad was, as you might imagine, not an issue...
BTW, anyone who missed Cathar's
Experiments in TEC based water chilling - go read that. Great stuff - got me interested in TECs again - thanks Cathar!