I was wondering there are any internal (cabin, cockpit etc) differences between a UH-1D that flew during the Nam era and those in the 80's or 90's. Sorry if this is a silly question given that helo's have the same designation

Not really. Some of the instruments, radios, GPS, etc. may have been changed or added, but they basically look the same on the interior. Newer ones (since the mid-'80s) would have a blacked-out cockpit with everything being painted flat black to be compatible with NVGs. Some had completely blacked out cabins as well. The soundproofing on the roof and rear walls is usually still gray though.

Also, all UH-1s built since 1967 have been H models, not Ds. They are pretty similar. The H was mostly internal (engine and tranny) upgrades. They do have some distinguishing features though.
D models had the pitot tubes and FM homing antennas on the nose. On H models the pitot tube was moved to the roof, between the upper windows; the FM homing antennas were deleted; and the FM "Towel Rack" antenna was added to the roof near the transmission cowling. Over the years, the external antennas and fittings have changed as well. Best bet is to find a picture of the helo you want to build and use it as your reference.

UH-1D



Vietnam Era UH-1H



Later model (1980's) UH-1H with wire strike kit, VHF/FM antenna (long whiskery antenna on roof), and intake particle separator.



Late UH-1H with windowless forward crew door.



Modern UH-1H SAR from White Sands Missile Range, NM. I flew on this helo last month. Notice the particle separator on the engine intakes, wire strike kit, APR-39 antenna bulges on the nose, and VMF/FM antenna on the roof. It had a totally blacked out cabin and cockpit.



Good luck.