is it possible to beat the caprice in my supra? if so what mods do i need to win?

While the other guy isn't looking, pull 2 of his plug wires.

Otherwise, the Caprice will toast the Supra. It is approx. 2.5 seconds faster in the quarter mile.

Unlike later Supras, the Mark I Supra, like the '78 was not intended to be a performance car. It was a personal luxury car, and does not lend itself to mods.
Short of swapping in a turbo six from a later Supra, I do not know anything that would make your car than much faster.