I wound up replacing a T400/NP208 with a NV4500. The truck was an 84 K3500. Because of the straight axle I did the following;

Had to move the tunnel opening back 3-4" - I had the tranny mounted on the engine and tilted up to meet the tunnel. Kind of made my marks. I had to fill in some low areas from the original indentation for a 465. I also figured in where I would need to make the cuts for the transaxle. Went pretty smooth but don't expect to put 10 and older kids in the middle of a bench seat with this mod.

I installed a 85-87 clutch/brake pedal assy. Bought it off of car-part.com. My tranny was a '98 so I wanted to stay with the interanl slave cylinder. The clutch master cylinder was a 85-87 unit but I also bought a complete 98 cluctch system. I only needed the metal hose but you can't get them separate from GM. I had a hose shop cut and braze the two systems together. The reservoir was also a 85-87 item but new from GM.

I liked the setup with an internal slave since the front drive shaft and exhaust made it pretty difficult to work on after everything was installed.

I spent $10-15 on advanceadapters for a couple of catalogs/install guides. You should be able to find all kinds of install info on the web.

The new clutch/brake pedal assy will require you to get way up under your dash. It has two additional support bars that are used to mount the clutch master cylinder. You need to find two small dimples (looking from front of truck) up and to the left of the firewall wiring harness connection. Up and to the right from the steering column. Can't recall which way the holes are oriented. Bottom left - top right or the other way. Don't get to crazy when you cut out the hole in the firewall for the clutch master cylinder. Since you are going to this trouble I would also recommend changing out your hydroboost unit if original or questionable. Try Hydratech brakes. They use a 96 model but set it up for you foolproof.

Since you already have a 4 speed you don't need to worry about the wiring harness for the clutch reverse switch. Don't know your feelings on clutch safety switch but the 84 won't work on the 85-87. Tried it and threw it away. I was the only at the time driving it so no big deal.

Go GM on your shifter and shifter boot assy's. Makes for a nice install. When you finish adding oil through the top ensure you use proper thread sealant on the tranny cover boot. mine backed out and egg smell in the cab for weeks.

I also called LUK clutch and they reasearched what I needed for my install. Was different from their website. It was done this way since the NV4500 didn't have an application for the two piece seal 6.2L crankshaft.

Good luck,

Jerry