If I recall correctly, some Dana 60(?) rearends for 3/4 ton had 10 bolts on the carrier cover, but are not to be confused with the GM Corp 10-Bolt of S/10 fame.
Hydraulic lifters were stock. There are guys racing 250s, so you can get solid lifters for them, but I would not suspect that of a farm truck. L6s do sound clacky, especialy if sitting for a while.
250s are OK, I have no experience running them. Too bad. I was gonna make you an offer for it, if it was a 292