Can I use a 6 spd Shimano freewheel on a suntour GT hub that originally came on a bike w/a 7 spd freewheel? thanks!

If they are both freewheels then the answer is yes. Roger

I think I read something on SHeldon Brown's site that said a 7 spd hub is not compatible w/ a 6 spd freewheel so that's why I'm asking to make sure

A 7 speed free hub isn't compatible with a 6 speed freewheel. Putting a 6 speed freewheel on a hub that originally had a 7 speed freewheel isn't a problem. I did it myself last Friday. Now if you have indexed shifters that will pose a problem as they must be matched to the number of cogs.

You should be able to use a 6 speed freewheel with 7 speed index, you will however need to adjust the upper limit screw of the derailleur so that seventh click doesnt throw the chain into the spokes. If the rider is using down tube shifters that can be switched to friction mode then he may possibly be able use 6 or 7 speed gear set...though the use of the 7 speed gear set with 6 speed shifters may be limited by how far around the DT shifter will go.

If you have an indexed Suntour system it may not play well with the Shimano freewheel either due to differences in cog spacing.

Friction systems do not suffer from this compatibility problem

I've tried that and could never get it adjusted to shift properly. The spacing was a bit off.

it's just friction shifting on this bike. It originally was a 6 spd freewheel on a 27" wheel but I'm converting to 700c wheels (which originally had the 7 spd freewheel on them) but would like to use the 34t 6spd freewheel that I just picked up for $7 on clist.

For not much more than that, you can get a Shimano 14-34 7spd free wheel.

If the wheel has a quick release you may have a problem with the axle length, or you may not. Replacement axles are inexpensive anyway.

Al