67 Cuda: disappointing dyno numbers, need some advice!

My 2 cents FWIW.
That's pretty good numbers on a chassis dyno as yellow rose already said. It's not dying at 5300, so your shift point will be around 5600 or so..
I don't beleive an intake manifold is going to make a big change. I know you're not satisfied for your purposes which I'm guessing is mph at the track. In which case maybe a street dominator with spacers will be some help with cam and head work.

Heads. Need to have valve cuts that will help the flow. The old approach was to use the larger valves and that still works but I've seen some impressive results done with smaller diameer valves. The problem here is no one knows.

Pistons. Remove a spark plug that's easy and look in. Many times I've been able to see enough to figure out what piston is it by the shape, valve notches and numbers.

Rockers. Don't see anything to be gained here with rollers.

Timing at WOT. Timing is RPM dependent. Map this out. In other words, check and plot the timing from slow idle (hot) to at least 3000 preferable 4000 rpm. Be careful! Just need need a couple measurements at the high rpms to be sure the timing is steady or slightly climbing.

Fueling. It would be good to have the AFR plots for the dyno pulls. Mixture should stay steady when at full load.

Compression check. You'll need to crank it through at least 3, probably 4 cycles to get the reading. Throttle open is best.