Look up the gearing in that GTO transmission. It isn't the best for a turbo setup IMO. the 2.66 1st gear style '98-02 Fbody transmissions would be better or if you can afford it a new T56 Magnum which is 10x better in all ways.
A stock pre '05 engine has LS1 style rods which are good to around 550-600rwhp before they bend. With a perfect tune it will last.... but in the end the rods will bend. LS2 style rods seem to be good to around 800-900rwhp.
A stock intake and TB will meet your power goals easily. A single turbo would meet your goals easily but twins are aesthetically more pleasing... and more $.
If you don't plan to AutoX or Road race your car you probably don't really need the aluminum block. Crunching the numbers with the power you want... it may be best to sell that LS1 instead of building it up... and get an '05+ style engine.
__________________
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....
|