Here is what is needed to put a gauge together thanks to Swamps site.

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.