'85 GMC S15-4WD, 2.8L 4 SPD Transmission

[pullmyefinger]

I have a B/W 4 speed manual transmission in this thing, and people are telling me my mileage would be alot better if I put a 5 speed in because of the overdrive.

1. I was told by a local dealer that a 5 speed tranny would bolt up to where the 4 speed was with no problems?

2. What about the transfer case, does that need to be adapters? If so, what are they, where might i get them, etc.?

3. The only bad part about that is that those trannys are very old and i may be lucky to fine a working one and/or have to have it rebuilt very soon?

4. Since my 4 speed BW works fine, should i just forget about the 5 speed and maintain the 4spd till it dies?

Thank you for any comments you may post.