Yes, you can use the S10 tailshaft. I recently rebuilt a V8 T5 and used the S10 housing and shaft. To do this, you need to completely disassemble both trannies and reuse the gears from the V8 T5. Also check the condition of the snout on the tailshaft for ANY wear or discoloration. Also check the inside bearing surface on the input shaft for the same. If there is any wear, the parts are junk and you will need to buy new ones. I know of a fairly cheap source for T5 parts if you need it.
Yes, if you make a significant change in rearend gearing or tire size from what was used on the donor tranny vehicle, the speedo gears will need to be changed. If the current setup reads correctly, then you would want to use the same number of teeth on the new setup.
One thing to remember with the S10 T5's is that the mechanical and electric senders are not the same or interchangeable. So if you are using a mechanical speedo, make sure the S10 tranny had a mech. sender to begin with.
