[N0DIH]

An odd thing with turbo engines I have noticed over the years. They all seem to have small valves. I have never seen a factory production turbo with large valves.

My 301 Turbo: 1.75 Intake 1.5 Exhaust (my particular car, 290 RWHP)
GN Turbo: 1.75 Intake, 1.5 Exhaust (245HP stock, easily to make 400+ with stock long block)

Honestly, with the boost, valve size may not matter at all. I guess if you are going all out, 15+ psi boost, sure, the valve will become a restriction to the flow, raising boost when it shouldn't be (backpressure in the intake tract will make the turbo back up and activate the wastegate early, thus lowering overall cylinder boost pressures)

Maybe increase exhaust valves on the exhaust, but until boost gets up, don't mess with them. BUT, OTOH, boosted engines often have sodium filled exhaust valves. The 301, not sure, they are fat suckers, enough room they probably do, the 3.8L Supercharged Ford, the 2.3L Turbo Ford, both do. Not sure on GN, probably does.

So, do consder that. If you are swapping valves, look for the necked down valves that have more flow around the valve, but keep with the Severe Duty versions if at all possible. Turbocharging is serious business. (more for Phase 2 not Phase 1) High mileage valves can be a risk.