I wouldn’t put the old ‘U’ bolts back on because generally speaking they are not good quality in the first place and because they tend to stretch when originally installed, both of which will shorten their life.
Equally it is difficult to get any real build up of zinc on the threads so stress corrosion could occur even if the exposed part of the thread looks OK.
Get a set of bolt crops (£18) from Machine Mart, they are guaranteed so if you break them they will replace them. Easy to use and bolt crop won’t produce sparks to damage your boat
Des