Engine bolts are usually UNC, and I suspect body bolts & suspension will be UNF.
Rocker covers are probably 1/4 UNC, inlet & exhaust manifolds 5/16 UNC, Bell-housing either 3/8 or 7/16 UNC.
It's always handy to have a selection of 1/4, 5/16 and 3/8 UNF nuts, bolts & washers.
(Washers are more-or-less the same as 6mm, 8mm & 10mm)