The string should be fine at 70lbs and nat gut also should not be much risk to the frame, like a stiff poly that does not flex. The shop is just playing it safe so they do not have to insure your expensive gut incase of break because this tension is out of the normal playing (stringing) range.
I would also recommend trying a hybrid with your gut in the mains and a poly in the crosses. This will provide much more control and limit the power of the gut. I played all gut for many years and now like hurricane or rpm in the crosses. These hybrids are also very popular on the tour.
You will also save some $$ on your string jobs.
For more info on max gut tensions, check out this TT thread: