The MN8 RPO is the 4l85e trans, they came behind 8.1's in Avalanche's and Suburban's, some 4.8's and 6.0's in vans(mostly the cutaway's) and the Duramax van. Do not get one with a parking brake on it as it will have straight cut gears(weaker and noisy). I would avoid the 91-93 models as they had some unique parts in them.
As to converting a 2 wheel drive to 4x4, it could be incredibly easy, or a bit more work. If it's one with a fully splined output shaft, you can cut it with a cutoff wheel, bolt an extension housing on it, and go. If it is setup for a bolt on yoke, then you have to swap output shafts. FYI, all 2 wheel drive 4l80e outputs are drilled(learned this one the hard way), you have to pull the housing to check it. Since this is in a swap, I would get a 4l80e with both an input and output speed sensor. 94+ 4x4 4l80e's do not have a reluctor installed in them for an output speed sensor, they used the transfer case mounted sensor. Sonnax sells a replacement reluctor that is fairly cheap, so you can add it to your trans if needed. This allows you to run whatever transfer case you want and not have to run one with a 40 tooth reluctor and magnetic speed sensor.
As to clutches, the raybestos and alto red's are said to hold well, but if they get slipped just slightly, they're toast. Everything I've read in the LS forums and other places says the late model borg warner high energy clutches are the best all around for holding as well as longevity. Dual feed the direct clutches internally, put the heavier direct clutch springs in, put a thicker intermediate snap ring in it, use the sonnax self regulating boost valve, a tcc valve with a seal(transgo has one with stock lockup, firmer lockup, or on off, sonnax also makes one), the sonnax over-run kit Mike talked about only works on 97+, do the transgo afl valve, as well as all the bushings(I use the sonnax wide stator support bushing). I would rollerize the output and forward hub, but it's not neccesary. I'm sure I'm forgetting some stuff, but these are some biggies I've picked up on from the few I've done.
As to the HD2 kit, the pressure relief valve they include is not the best, and has proven to be slow to open if it does open for high line pressure. I know I had problems with high line pressure on the one I did at Christmas that ended up being the boost valve, had 80 psi idling in D, went to 175+ in reverse(max line pressure, pump whined like crazy), putting stock springs in the boost valve made it acceptable, but still not what I would have liked, and the sandwiched separator plate is a nice workaround to dual feed the direct without teardown, but if you have it apart why not do it internally instead of a workaround.
