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Singlespeeding it was no problem at all. I just used a regular 135mm cassette rear wheel and some spacers and eyeballed the correct chainline. I may even have measured it, don't quite remember.
26.2 was the correct seatpost, but that varies. Try googling a datasheet of your specific model, GT went through quite a few sizes.
I actually still ride this with the crap seatpost and the ultra-high stem. oddly comfortable.
I have a GT Timberline in the same colorway. Makes for an awesome city bike.
Bulletproof it is.
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