t1s10 is trying to say that there are a lot of variables contributing to mileage including rear gear ratio and the gear ratios in the 700R4 along with the shift points in terms of rpm and in the electronic version this has been supposedly optimized in the factory computer for a given set of gears and the torque curve of a given engine. Sometimes you can get lucky and everything transfers OK when the engine is changed but you need to compare the gears of what you have now to what you will have with the modified setup. On the other hand a four speed gives you more flexibility and the OD should give good mileage. In addition the low gear of the 700R4 is lower than many other transmissions so you should have low end torque multiplication as well as good mileage from the OD fourth gear. Then if you modify the 350 you need to select your cam and intake to make power in the range of your gears and shift points. Try

http://home.tampabay.rr.com/1bking/c...calculator.htm

to check out the rpm ranges of your gears. With the V6 your rear gear ratio may be pretty low for the 350 and the already low gear of the 700R4. You may not be off by much using the rear you have now because of the wide range of the four gears in the 700R4, but it might be a good idea to check AND use the electronic harness and computer chip the 350 was set up for. Hopefully the range of gears in the 700R4 may cover your new setup but the rear gear may be too low. Try running the calculations and try to get the electronic harness as lt1s10 suggested.

Don Shillady
Retired Scientist/teen rodder