Driveshaft: how much clearance?

Shop here wanted $100 just to balance check a shaft. I'd rather put that towards a brand new one.
I shortened a shaft- custom length for the old Dodge I built, hoping to luck out. It talked to me above 60-65 mph, so I finally got it balanced, plus straightened for a little less than that. Soooo nice to get above 70 without cringing :)