No one seems to be jumping in with suggestions, so I'll throw some ideas out there - but also say that 'cheap' is a relative term.

Champion Radiator has a number of aftermarket aluminum, down flow radiators. You should be able to find something that will fit with in/outs in the correct location: http://www.championradiators.com/

There are a number of options on ebay.

Summit racing sells universal cross flow units under $200 but may require you to fab mounting brackets of come up with a mounting system. I used one for my blown BB Duster for clearance reasons. http://www.70duster440.com/cooling.html