Len
I would have thought it was a good idea to put larger valves in the power pack heads and use them on the 350. The higher compression would make it seem like a good way to boost your HP.
It's a good thing we have Richard on this forum to explain things like this.
Thank you Tech1
The only problem I could think of is that the 283 heads don't have the boltholes on the ends for the 350 accessories.
The good news is power pack heads bring a good price on ebay.