Just buy a cheap 39 tooth ring for the crank. Derailleur should be designed to handle it just fine.Joe the biker said:A buddy of mine and I started riding recreationally last year. I caught the bug harder then he did and two bikes later I have a light 10 speed. My buddy is still pushing a 20 year old 6 speed. He has alot of trouble with the inclines (not long or steep hills). I looked at his bike and he has a Shimano 600 drivetrain with a 52/42 front and a 25T as his biggest cog in the back. Will the Shimano 600 FD work with a 39 inner ring? If not can a 105 FD be installed on a bike with down tube shifters? Are there 6 or possibly 7 speed cassettes or free wheel available that would have a bigger gear. I am not sure whether it is a freewheel or freehub.
He doesn't want to spend alot because this will be a temporary fix. If his work picks up he is getting a new bike and if not he hopefully will get in better shape.
Yes, a 105 derailleur will work with DT shifters, no problem. It'll actually work better because you can trim a friction DT shifter. You can't do that (as well) with STI.
And yes, there are cheap freewheels out there that will work. Nashbar usually carries them. Avoid the mega-range stuff, though. His rear derailleur will probably max out on a 26 or 27 tooth freewheel.
Figure if you have the tools and know what yer doing, you could make these changes for less than $50. If you've got to go to yer LBS, figure $100. Either way, pretty cheap.