If you have an all iron 355 swapping to an all aluminum LSx engine would shave about 200lbs off the front end. That alone would make a huge difference. An engineer would give his left nut for that improvement
Not knowing what compression your current engine is.... could also be a big factor in throttle response swapping to a new one. An LSx engine can easily support 11.25:1 on 91 octane which makes it pretty damn snappy on throttle. 93 octane can support 12:1 compression. The '99+ LS1 is worth buying. The older ones weren't as refined IMO resulting in piston slap, oil consumption potential, non-improved oiling system like the '99+ style. A simple cam/valvespring/tune swap will produce an easy 400hp+.
A modern transmission (not a 4L60E/65E/4L80E) that has 6 gears would make a huge improvement as well. While the above mentioned transmissions are fine..... they are antiquated in reality. A 6L80E/6L90E is rather awesome.......... or even a T56 Magnum 6speed manual!
It all depends how wild you want to go. The sky is the limit
An LS1 with ported heads, cam/dual valve springs/LS6 intake, headers, tune could easily make 450hp and be super driveable..... if not closer to 500hp flywheel with a more aggressive cam. Run as much compression as your octane allows.
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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....