350 HP from a 326 is a tall order! A "high" rev range will be needed. Porting and a "big" cam will be required. Larger valves can probably help some, but not the "standard fare" for the Pontiac, since the bore size is too small for the 2.11 intake valve to "work" properly. The 1.96" intake valve is probably the best.

The connecting rods need to be replaced. The original cast rods weren't "good" when new, and are now over 40 years old. For a little more than the cost of resizing the stock ones with new bolts, you can have forged rods.

326 is a "small bore/long stroke" design. Trying to make it perform like a 327 is basically futile. Real "torquey", though.

If you wanna get "silly", I can outline a good build stategy. Not gonna be "cheap", though.

Jim