Alrighty then, here is the writeup I've been talking about:
First truck = 1980 K25 originally equipped with a 350 / 32 spline SM465 / NP208 and retrofitted with a turbocharged 6.2L diesel / 93 NV4500 / 32 spline t-case.
93 / 94 and part of 95 NV4500s have an external slave cylinder much like the hydraulic SM465's use. One major problem with these older transmissions is that the clearance between your front driveshaft and the slave cylinder is extremely tight. My truck is lifted 6" up front, and it only has about 3" of clearance. One advantage to the earlier transmissions is that the master cylinder for those early trucks is very easy to retrofit.
After having driven both gear ranges of NV4500, and taking into consideration the clearance issues, I'd very highly reccomend that you grab a 95+ NV4500 with the internal slave cylinder / throwout bearing assembly. I intend to put a newer transmission into my K25 next summer.
Bolting the transmissions in is extremely easy! They are almost the exact same length as the original SM465 / NP208 combinations are. Within 1/4". They even use the same transmission crossmembers / mounts as the 465 / 208s do. I chose to install an NP241 for a slightly better low range on both of my trucks, but you can install the 32 spline NP208 onto an NV4500 without any adapters of any sort. If you use the 208 or 241, you will NOT need to change out your driveshafts.
The two "difficult" parts of the NV4500 swap lie in the floor hump and in the hydro clutch. The shifter stub shaft for the 4500 sits in a different position than a 465 does, which means you have to close up the original 465 shifter hole and cut out a new one for the NV4500. Unfortunately, the NV4500's shifter sits extremely close to the 208 / 241 shifter and makes it nearly impossible to install a shifter boot. My solution for this problem is going to be a floor shifter from a 00+ truck installed on the driver's side of the hump with a custom built cable running from the shifter assembly to the shift arm on the t-case.
The other difficult part is installing the master cylinder from the 88+ truck into your truck. I personally used a knockout punch set to make a 1" hole in the firewall, placed the master cylinder in it and drilled out the two mounting holes. I then placed longer bolts through it and made a pair of support braces that run up to the hydro pedals. I then took a 1/2" shaft bolt with an un-threaded shoulder, drilled a hole in the pedal at the appropriate height, installed the bolt with a nut on either side of the pedal, then cut the head off. The master cylinder's eyelet fits right over the end of the bolt shaft. Drill a hole in the side of the bolt for your cotter pin to holt everything together. If you are careful the master cylinder looks totally stock and functions fairly well. Only thing I noticed is that the pedal bottoms out a wee bit early, so I may have to shorten the length of the master cylinder's pushrod.
The second truck is a 86 K15 originally with a 6.2L diesel / 700r4 / NP208 then was retrofitted with an 06 NV3500 / 32 spline NP241.
This truck was also a very easy swap. Despite some trouble finding the correct flywheel for the engine (still haven't got it right) everything again bolted directly into place. The NV3500 is the exact same length as the 700r4 is, so by bolting a 32 spline NP241 on, I was again able to use the same driveshafts as before, and as with the NV4500, the NV3500 used the same transmission crossmember & mount as the 700r4 did. Infact, I used the tranny mount bushing from the 06 truck with the 86 crossmember, the spacing was all exactly the same.
As with the NV4500, the NV3500's shifter sits a bunch further back than the other stock manual transmissions, which again presented the problem with the trasnfercase shifter clearance issues. I went to the wrecking yard and found myself a shifter boot from a mid 90's Suzuki Firefly and was able to use it with the NV3500's shifter. The clearance is still very tight, but it looks ok and works just fine.
There are also internal / external clutch slaves for NV3500s. The 88 - 95 units all run external, which also presents the major clearance issues, while the 96 - 06 units all run internal slave / throwout bearings like the newer NV4500s. The 96 - 06 transmissions remained unchanged despite the change in truck bodystyles in 99!
Since my transmission is from an 06 from behind a 4.8L, it has an internal slave cylinder. Since the master cylinder is way different between the 96 - 99 / 99 - 06 trucks, I chose to replace the entire assembly with stuff for a 96 K1500. The master cylinder presented a pretty major problem as compared to the older external slave units that bolt on as they utilize a stiff metal plate behind the firewall's sheet metal that the master cylinder locks into.
My solution was to purchase that metal plate assembly from GM for approx 60 bucks Canadian, and cut out the parts that I needed and bolted it onto the inside of my firewall. I then had to slightly file the mounting slots on the master cylinder due to the increased thickness of our truck's firewalls as compared to the newer trucks. I modified my hydraulic pedal the same was as I did for the NV4500 with the 1/2" bolt.
Despite having some clutch problems with the 6.2L in my 86 (flywheel is not quite right, but I figured I'd try it anyways since I'd already tried two others that wouldn't work at all) it has been extremely nice to have the 5 speed in there!
The turbocharged 6.2L with the NV4500 has a smooth acting clutch, but I find the gears are a wee bit far away for a diesel. They'd be great with a gasoline engine though. Both transmissions shift very quickly, and easily compared to the older SM465s. They have short throws, and are far quieter all around.
