One of our pattern makers wrote up about the installation of the S10-T5 in his buddy's truck:

It all started with Scott's poor old '57 Chevy truck. You see, he dug what loosely resembled some rusty truck parts out of a field somewhere and managed to cobble it together into his daily driver. He now has put ten-thousand miles on it getting 17+ mpg while driving like a madman (trust me). The truck sports a 350 and 69 3.08 geared Chevy truck rear end but he's got an '88 S-10 5 speed in it. The problem with the Borg Warner 5 speed is that compared to older GM transmissions the input shaft is too long. To get around this people have been cutting inputs shafts down or using a stack of washers to space the tranny back. Both of which are the mark of a hack. When using washers Scottʼs problem is that the input shaft is not centered to the crankshaft pilot bearing by any precision means at all. this can be catastrophic and eat the input bearings alive or eventually cause the input shaft to *break*.

I finally convinced him to install the S10-T5 and prevent the inevitable.

We started by pulling the driveshaft and transmission tunnel cover, disconnected the speedo cable, removed the shifter from the transmission and then dropped the tranny. Don't forget a drain pan as fluid will pour out the tail shaft hole. You can tape a rubber glove over the hole or use a spare yolk to plug the hole if you want.

With the tranny on the ground we noticed that there was a large amount of RTV that had squeezed out from between the input bearing retainer and the case. This had to be removed otherwise the adapter wouldn't sit flat. This is also a good time to get the correct speedometer gear.

I found it was easiest to put the adapter on the transmission first as it was a slight press fit. Now you can stab the two together into the bell housing. When swapping from an older tranny to a T-5 you will need to use the clutch pressure plate that was designed for your flywheel but, you have to use the clutch disc that was designed for the transmission. Take note of what vehicle your tranny came out of.

With the tranny back in and all bolted together we removed the fill plug and filled the transmission with ATF from the top through the shifter hole until fluid ran out the fill plug. Next, replace the fill plug and shifter with a fresh bead of RTV.
You can also use a fluid called Synchromesh by GM but it costs around $28 a quart and will make it shift smoother. Amsoil makes a similar product for around $8.

You'll notice that this truck doesn't have a transmission cross member. They never came with them. In this case it's actually alright, because the engine is solidly mounted with 2 mounts in the front and 2 that bolt into the bell housing.
Lots of drag cars with motor plates don't use a transmission mount because the chassis flex will crack the case. I figure if it's ok for a car with 1200hp, it's ok for our truck with 250hp.