'68 PW weighs north of 4000 lb, lot of truck for the engine to pull around.
Your stock cam has something like 178 intake duration at 60 and .378 lift. It'ws really small.
Even a stock 360 cam, which is something like 199 at 50 and .4 01 lift would be better.
Years ago Comp made a Magnum series cam that was 204/204 .425/.425 on a 110 LSA which I think would be perfect. I believe it was 252 degrees advertised duration.

Smallest Voodoo cam or get with Hughes. I wouldn't go over 209 intake duration and try to keep the LSA tight for more midrange.

I wouldn't have guessed that 0.441" valve lift would cause problems, but the stock springs are pretty wimpy.

A set os stock springs from a 302 head 318 roller motor would be a step up. I don't know about coil bind, but they are noticeably stiffer than flat tappet 318.

I've got a used set I'd give you for the postage.

R.