Those early engines IIRC were already 8.5 Scr., and the pressure at sealevel mighta bin in the range of 125 (+/- 15)
But at 5000 ft the Sealevel pressure may be down in the range of 90>120psi
In other words, there may be nothing wrong with your current pressure at 5000 ft elevation. You need to do a LeakDown test to see how bad things are leaking..... if at all.
To get the pressure back up to say 140psi, the static would need to be ~9.5, but 140 is still pretty feeble.
To get to snappy, would be 155psi, and with the 50*Ica , that would take ~10.2/1
Good luck making that happen on a modest budget.
And even if you get the pressure up, finding air to burn at 5000 ft is still gonna be a chore.
And then you may have to deal with detonation.
Do yourself a favor, install some performance rear gears like maybe 3.91s and be happy.
Or supercharge it, as already mentioned.
or just bite the bullet and install a V8, and because yur at 5000, my vote is a 360. any 360. Even a tired stock 8/1 360 will be a minimum 50>60% more powerful below 3000rpm, with NO mods at all.
Will it break the 903?
Well that will depend on how you drive it. But probably yes, lol.
But so will a supercharged 198.
If you like,
I can ship you a couple of pallets with any size SBM core on it, a 4-speed, and an 8.75 with 3.23s, and a reconditioned sureGrip, and almost everything you need to make the swap except no exhaust, unless you want to run standard A-body logs, possibly including 340 logs.
Or you can jump in a pick-up, and come and get it, and we'll sit around and bs for a day, or two.
I got two empty bedrooms.
I don't to beer, but I got plenty of Pop.
Bring the Duster and a trailer, and I'll rig up an LD tester.