How to make Horsepower?

The head flowing ability is the real limiting factor it's hard to build any real N/A power out of a /6, 6 cylinders head need to flow 33% more to be able to make same power as an 8.
300hp /6 would be an major build, your hp is in the low 100's a little work might get near 200, anything above that you need head work.
Saying that a 2bbl, exhaust and mild cam(any thing got to be a major step up from stock)
and as younggun said gears, personally I'd swap in a 8 3/4 with gears first for now with an eye to future of putting an 8 in but you do stay with /6 a built 6 still needs a strong rear end anyways.