Actually, there are more tube frame rear engined speedster kit cars than there are pan based ones (at this time).
If you build a 550 spyder, they are already tube frame for mid-engine. Here's an affordable way to build a really cool one:
Take a rusted out 914, use the suspension, steering, steering column, brakes, pedal assembly, shifter, shift linkage, instruments (refaced for an authentic look), engine, transmission, axles, parking brake and wheels.
Build the 550. When we built our 550, we used an early 911 as a donor and got the transaxle/shifter/linkage from a 914. It would have been less expensive to build from a 914 base.
The black one in this picture is ours. One of the silver 550 cars is a subaru engine.
angela