The only differences between a CH-53D and a HH-53C is the Engines, paint and armorment. CH-53D's use T-64-GE-412's or -413's. HH-53C's used T-64-GE-7. All made by General Electric. (GE brings good things to life.)
The Air Force liked to mount M134 7.62mm miniguns at their right side crew door, left side A/O window and on the ramp. Marines would mount a couple of .50 cals, one at the crew door and one at the A/O window.
I have built Italeri's CH-53E in 1/72, and I have Italeri's HH-53C in my "one day it will be built" pile. I'm not impressed by either kit, but both can accurized with a little TLC and some good reference material.
The Italeri Airforce HH-53C could be made into a Marine Corps CH-53D simply by adjusting paint scemes and finding some decals.
I saw on your site links that you build fire fighting equipment and vehicles. Are you doing one of those CH-53D's that were fitted for fire fighting duty? That would be cool.
Semper Fi,
Chris