being an 89 he probably has the D35 rear. so do as weazle say's and make a junk yard run for an upgrade (D44-CH9.25-Ford8.8). if he has the antilock, he may have the better HP D30 with the larger joints and non disco. if so you might want to go ahead and re-gear that one. if it is a LP D30 then back to the junk yard to find an HP D30.
and from what i keep hearing, jeeperjohn and robert are the go to guys for gear set up.