you can defently fit 38's if ya cut the corners on the front fenders speacially if your not gonna wheel it hard. . .
my 78 (same openings) i had 39's on it with 4 and 3 and then 44's with alot of cutting

this was 4 and 3 with 39's (stupid sts's . . only on there a week lol) *tear* it was such a small truck then