If you don't have access to a scantool, then go to your local hydraulic supply house, and have them make you up a mechanical gauge. You might spend $60 or so on high pressure hose, fittings, and a quality 0-3500psi liquid filled gauge....but having this hose available for future diagnostics might be more valuable than you think.
The hose which you will need will need to be about 40" long, rated for (minimum) 3000psi working pressure (12k psi burst rating!) with the gauge on one end, and a #6 female JIC swivel fitting crimped onto the other.
You will also need an individual fitting to screw into the head to go from the head to the hose.
This single fitting will be a 90degree fitting # 5 "male boss" (sometimes called # 5 o-ring) on one end of the 90, and a # 6 male JIC on the other end of the 90. Our local hydraulic shop would label such a fitting as 5MB-6MJ90
"JIC" is nothing special...it's just 'hydraulic talk' for a 37degree flare fitting...standard hydraulic stuff here...nothing rare by any stretch.
Looking at the top of (either) cylinder head, you'll see the factory stainless braided oil lines (one to each head) then you'll see a few bronze colored plugs... (Engine off, of course) Using a 5/8" wrench, remove any one of the bronze plugs, and install the 90deg fitting into the hole. (save the plug for reinstallation , after testing). and the O-rings are reusable, unless brittle, cracked, etc...
The single 90deg fitting will have a "jamb nut" on the O-ring side...screw the fitting into the head, "aiming" the 90 away from the turbo, intercooler pipes, etc...and then tighten the jamb nut, to "squish" the O-ring also locking the 90 from "spinning" around as you attach the swivel end of the hose to the 90. No sealants, loctite, or teflon tape are needed on "JIC" or O-ring fittings...and don't overtighten them...usually about 180degrees with a wrench past finger tight...check for leaks prior to getting too far from home...
run the hose, away from moving parts, electrically conductive junk (like batteries, glow plug relay, starter relays, etc) ..and just route the hose up through the cowl toward the windshield...for short term testing purposes, we just lay the gauge up under a windsheild wiper, and go drive...testing it just like I describes previously...chip in the highest setting, floor the truck, etc....
If you can maintain 2800+ psi of ICP, then any injector 250cc's and under will perform exceptionally well.
If you can maintain 2400psi then that's not bad, not great either.
2200psi is mediocre, and full performance from any injector will not be achieved with that HPOP, though power WILL go up with nearly any injector, driveability (excessive smoke) might be an issue.
If you cannot maintain 1900psi, you might seriously consider an aftermarket dual HPOP system, or our Gen3 HPOP. Nearly any larger injector is going to make the truck feel sluggish, and smoke excessively.
