I didn't use a universal tap and die set. I bought specific bottoming taps from tool & die companies. They were a very good fit. Maybe you could tap your blocks to the next size up?
try using form taps they work much better than a standard tap and a lot more accurate sizing.
when using form taps, make sure you use the correct size drill, as form taps require a larger size drill than cut [regular] taps of the same size thread pitch.
