right now u have a TF904 (30RH) with the final gearing 1:1. if you change to a 5 speed, that would include an overdrive and reduce your rpms. if you ran an AX-5 (this is the stock 5 speed for the 4 banger), guessing that you have the stock tires and gearing, you would be reving 2300, roughly. if you ran an AX-15, (which was the 5 speed for the 6 cyl) you would be reving at 2100.
this is a good web site to help you with the gearing and rpms
http://novak-adapt.com/knowledge/gearing.htm
as for the thrust bearing, idk. i think as long as you get the right flexpate for the tranny, i think u should be alright.
hope this helps.