1st post, R154 /R151f gears compatibility question

[MattInTheCouv]

Howdy everyone, just registered here, and it seems there is a lot of in depth technical knowledge that i haven't been able to find at 'toyotanation'. so, my question is if the 5th gear from an r154 transmission could be put in my r151f tranny during my upcoming rebuild. i would really like to lower 5th gear. there just isn't enough of a drop from 4th to 5th. my research has shown that the 5th in my tranny is about a .83, and the 5th in an r154 is about .75, this would definitely help with gas mileage and excessive engine RPM and wear at high speed. is this do-able?

thanks!

matt