i have used a 97 trans in a 98, and a 00, in 99 in a 98 as well, and never had any problems with the speedo. you do have a speed sensor in the diff, but i have heard there is also one in the case, but as i said, i have never had any problems. all of them where 5r55e's. i have also changed the revers ban in one as well. getting inside a trans scares the hell out of me, but once you have a manual infront of you (there is a thread on here that you can down load them) it really wasnt that bad, and took about 1 1/2 hours to change. i didnt need to rebuild it either.
one last thing, both SOHC and OHV motors use the same type of trans. they both also have the same bolt pattern. a 95-96 trans will not work however. they are a 4r55e