engine dies after idling for 5 minutes, then starter won't turn over

[Jerry Clayton]

SPF????

You have bad grounds---common in fiberglass cars/boats, especially cars with rear mounted batteries
Clean the frame where the ground attaches and also run a number 6 wire up to a starter bolt as an extra ground.
Blas , a WCCC member who posts here has wiring diagrams for the SPF cars that are serial number specific--get one
With bad or poor grounds it is possible to get back feeds in the electrical system that will give results a syou speak of

Also I have found on SPF that they call the grounds earth and some components (lights) are cross wired (tail lights/parking lights)

Jerry