1985 was the first year of the hydraulic clutch setup so the firewall should already be dimpled for it. You'll need to carefully drill the holes with a Unibit. Measure four times and drill once...

starting with a tiny pilot hole.
You may need a bolt-in high transmission hump or a mild body lift to clear the NV4500 with the trans tunnel.
An early 1990's 2wd Cummins Dodge NV4500 should have a gear drive speedometer setup. AND YES... You can use the Dodge 2wd NV4500. In this case it's probably preferred. The GM T400 chassis NV4500 only has VSS Speedometer parts not gear drive. The Cummins bellhousing to engine bolt pattern is on the Dodge bells GM is setup to mate with GM Corporate standard bellhousing to engine bolt patterns.
You'll need a universal joint to mate with the transmission output yoke at the transmission end of your driveshaft, the right length driveshaft (probably custom length but not necessarily), and you'll need to massage the transmission crossmember for the new mount.
That's about all the help I'll be. Other folks with more specifics about motor mounts, belt driven accessories, and firewall clearances among other considerations will jump in.
You may need to get heavier front coil springs to carry the much beefier Cummins... There are other folks that have done this swap that can tell you more on this and other finer points.