Summit has a GM Performance Parts crate motor that should fill your requirements nicely.
This one is an HO with forged steel crank, top of the line rods, 425 horsepower at 5250 rpm and 500 ft./lbs. of torque at 3250 rpm and 8.75:1 compression so it should run on 87 octane. $5000 seems OK for what they give you.
Jegs offers what appears to be the same thing for a little less money @ $4900.