the 2wd rte trans is just a v-6 trans same as in the 2wd v-6 trucks. it will have a high first gear same as the v-6 transmissions. find a v-6 4wd trans, run the rte bell, and buy the adapter to gear driven cases, it is way less work than you are planning.
if you want to you can use the trans you have, you will need the shift rails, extension housing and main shaft and shifter from a r-151 and you will still have like a 3.83 first gear which is worse than any 4 cylinder 5 speed trans that has 3.93.
