An LS2 or LS3 production aluminum block can handle 1000rwhp with a good tune on quality fuel. It'll even do it with a production crankshaft. Beyond that you want the 6 bolt style block/heads.
The LSx iron block easily weighs 120lbs more than a production alum one and they need a lot more machine work than some other aftermarket blocks. DART recently came out with a block also. RHS block is used a lot.
There are a few built shortblocks using aftermarket blocks for sale on ls1tech. The early LSx castings had core shift issues. Make sure you get one that isn't from that batch.
Any reason why you are going supercharger vs. turbo(s)?
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Project: 1969 Chevelle, alum 5.3, GTS76 turbo, Chiseled Perf A/W intercooler, Holley Dominator EFI, Davis TC, JRi coilovers, 14" brakes, T56 Magnum. CTS-V Recaro seats!
Coffey Fabrication Nashville, TN doing the fabwork! 800rwhp+/25mpg+/9's@150mph goal 
https://lateral-g.net/forums/showthread.php4?t=52976
1993 Mazda FD RX-7, built LS7, T56 Magnum, Holley HP, Davis Tech TC, 8.8" 3.73 Cobra IRS diff, DSS axles, Ohlins coilovers, big brakes.
CCW 18x11 front 315/30/18's and 18x12.5 rear on 335/30/18 RA1's, 2850lbs
10.31@137mph 165mph Shift-Sector 1/2 mile speed
441ci AllPro headed motor and 200lb weigh reduction in the works....
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