Strangely you're able to run your DeSoto-powered roadster in the appropriate displacement class it belongs -- Fuel or Gas, Modified or Street Roadster, front- or rear-engined. Have fun -- see how fast you can make it go. "I've got the fastest DeSoto-powered fuel roadster that ever ran at Bonneville!" {I'll help you get a trophy (like participating in soccer)}.
As I always am making fun of the motorcycle classes, the SCTA has it right. They don't give a damn of your old-fashioned Harley has two cylinders or whatever. One-cylinder, two-cylinder, four-cylinder -- 1000cc, you're in! (Of course there's now 28 sub-sections to navigate.) They've got some Vintage classes, but they don't say BSA, Harley or Parilla.
And there are a lot of folks that can probably out-spend and out-think you along the way. And they may have access to technology that's not approriate for your combination. But if you set your own goals and reach them -- be happy.