That 83 D250 was never on a race track, no ET available, but I had enough miles on it both before and after the head and cam swap and whether you want to believe it or not is your call. All I know is that it ran quite noticeably better after the swap than it had before. I doubt that that engine ever saw 4800 RPM. So that limitation you claim really didn't matter for how the truck was used.
I may have a chance to get it back, I sold it to my cousin a few years after the parts swap.
Maybe it wouldn't work for you but it did for me. Hauling and towing, was where I noticed the biggest improvement. About as far from racing as one can be.
It worked for me, well enough that I'd certainly do it over again the same way as the 1st time.
I didn't come here to brag, different things work for different people