[SCOTI]

Larger valves in the heads doesn't automatically equate to better just like a larger cam doesn't automatically equal better performance.

The right cam & heads (or re-worked heads) w/the right pistons (compression) are what will make power.

A plain-jane early 70's 350 would likely have 17-18cc dished pistons. Couple that w/the 76cc chamber heads & you have your basic low compression no frills sbc. Larger valves won't help compression nor will they help increase power much when there's minimal pressure supplying the mix. A larger cam w/more aggressive timing events would help things sound better (like it has power) but not boost perf by much if any. Too big of a cam w/low compression can actually hurt things when below the upper rpm range as low speed cylinder pressure is in the basement.

You want/need flat-top pistons & smaller chamber heads. Later model Vortecs offer superior flow + velocity vs any previous cast iron head & their smaller 64cc chambers help compression. You can spend money on the old 76cc heads but won't get the same results as you could swapping to better flowing heads.

It's all about the combo of parts. We did several 350ci motors years/years ago (pre-Vortec era) w/flat-top pistons & iron 76cc GM 882 heads (milled for true flat surface & to help compression). Each had a mild cam (~222° int duration was the largest cam) & made just over 300hp on the dyno (so 240-ish HP to the ground).

All worked well but I don't see you hitting a 300hp number w/o pistons & spending $$ on the heads. There are aftermarket alum heads that now offer much more bang for the buck & don't require the Vortec specific intake & related parts (which adds to their overall cost to use).

I recently picked-up a 69 350 that was plucked out of a fellow board members C10. It will get the same treatment & be the replacement for my '78 Malibu's low compression 305sbc. The dished pistons will be swapped out for flat-tops; some aftermarket/better flowing alum 64cc heads will replace the small valve iron units, & a mild hyd roller cam (basically the GEN2 sbc GM LT4 Hot-Cam) to upgrade the timing events. I'm estimating 300hp to the ground.