There is an optional gear set to make low gear lower in a TH350, BUT the parts alone are about $500 before labor AND then low gear is still not as low as in a 700R4, so even with the change and expense your low gear response will be less. Further, the TH350 mount is forward of the mount for the 700R4 so you will have to change the mount and the driveshaft may have to be longer(?). Overall it is just easier to repair the 700R4 and might actually be cheaper than changing the low gear in the 350, moving the mount and maybe changing the drive shaft AND losing the OD 4th gear in terms of lost mpg. Note that the 700R4 has a terrific low gear ratio, lower than many stick shift trans!

Don Shillady
Retired Scientist/teen rodder