The truck oil pan is gonna hang way past the frame, you'll want to find an F-body oil pan, pickup tube and windage tray to swap on before the motor goes in.
Either the 1" back plates or the standard type will work fine but depending on what the truck originaly came with you may have to elongate the mount hole in the center of the crossmember or drill a few new holes in the frame neither would be that hard.
I've got a set of the standard mounts for sale if your interested.http://www.ls1truck.com/forums/sale-...cers-more.html
1991 Silverado-L33 5.3L, 212/218 .557 .562 115+4, ASP under drive pulley,180* thermostat, 90mm truck intake w/ LS3 throttle body. Shoenfeld conversion headers and duel Flowmaster exhaust. Performabuilt LVL 1 transmission and 23-2500 stall converter.