Uponor (formerly Wrisbo) recommends limiting PEX runs to 300' so you should be ok.
I have installed PEX tubing in poured jobs and staple ups and worry about abrasion from expansion and contraction, but have not had a problem yet. Somehow I have always failed to as the question when in seminars and talking to reps'.
Being only a garage I am sure you will have the mixing valve set way back to only melt the snow and take the chill off so I don't think you will have a problem.
My own house is a staple up. Even though it creaks when starting during a cold start up, it is a nice heat. Everyone who has one also says so, although some of the installs leave you a believer in codes and standards. Some of the earlier jobs were pretty sloppy workmanship.
Uponor has a plant in Minneapolis MN (Lakeville or Apple Valley actually). Get one of their design manuals.
What is especially interesting about the Uponor system is their joint techniques. It is a plastic sleeve affair that contracts. Real neat, simple, and they the best - Maybe so!
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