what do i have?

What type of transmissions came in the 1987 s-10 with a 2.8? and will it bolt up to a 350?

Do you have a manual or auto? if manual, you have either a 77 mm or a 77.5. If the gearbox is bolted to the clutch housing, you have a 77mm. If the gearbox and clutch are one piece, you have a 77.5mm
If it is auto, you have either a 3 speed or four speed. three speeds came in 180c/200c, 4 speeds are either the 700R4, the 4L60, or the 4L60-Eif you have a cable that runs from the throttle lever on your carb or FI throttle bobyto your tranny, you have either the 700R4 or the 4L60. Whether or not they will bolt up I cant really answer, but I'm guessing that for the 2.8 the bellhousing would be the wrong size. My understanding is that a vehicle with a 4.3 l is usually the best candidate, as the swap is much easier, and a lot of people who have done this seem to prefer the 700R4 trannies. here is a link to check out
http://forums.s-series.org/viewforum.php?f=12

ok, the truck is an automatic, ill check on that other stuff tomorrow

You have a 700r4, and no, the transmission will not bolt to a 350. The 2.8 is a 60 degree motor, the 4.3 and 350 and 90 degree motors. They won't mate.

So im going to have to find a truck with a 4.3 in it and take the tranny out of it for this to work>?

4.3 or a 5.7

if the truck has a 2.8 no the tranny will not bolt to a sb 350, then you have to get one from a 4.3 truck or a truck that had a sb 350 in it, sometimes the salvage yards will swap tranny for tranny