Hi Im new here and I have a question what rear end came on V6 '90 mustang LX is it 8.8" ?

7.5 most likely. As far as I know no V6 had the 8.8.

They came with a 7.5.

they didnt make a v6 in 1990[&:]

if it was a 90s then it would be a 7.5 with either 2.73 (94-98) or a 3.27 (99-04) open diff

90 LX's if it had the 2.3 had a 7.5 and 3.45 gears or 3.08 gears depending on 5spd or auto

LX 5.0's had 2.73's or 3.08's