A pre-1935 anything frame, I'd say, if it's American. Many European bases right up to WWII might be suitable. What you want is a frame that will look good when exposed, especially around the front end. Best would be a radiator directly over a beam front axle, and a frame whose extention forward of the radiator is slender and simple. Of course, depending on legalities, you could build a frame from scratch, and then you might find '60s commercial-vehicle parts that do the job and look the part. Simplest way to get a project like this going would be to start with something like a mechanically sound, part-restored '33 Buick four-door, say, which might be got at a reasonable price. As in any other kind of hot rodding, it's about creativity and judgment.