It's always a good idea to have an aftermarket steel bell blanchard ground just to make sure that part of the drive train is correct.
That way if you do have a clutch issue or premature bearing wear you know what it isn't I have seen a fair number of posts on here about guys that have found the block face not square with the crank flange I checked my last 2 builds that I knew were going to be stick cars and they were both just under .002 measured with a "known good" bell and my own tool that holds the dial indicator very rigid bolted to the crank instead of a mag base unit.
My point being as long as you know the bell is flat and true you can trust your measurements at the retainer pilot
Gus