I want to swap LS6 5.7L but is that to big of a motor? how big can you go?

It depends on your mechanical and fabrication ability.. There are Fieros with 455 cubic inch big block engines squeezed in the rear..

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1988 Fiero Coupe 2.5L (Written Off in an accident in 2006)
1990 Trans Am GTA 5.7L

well there is a 454 sbc crate motor that should bolt in just like a normal archie kit. http://www.worldcastings.co...ods_pages/101011.htm

If its a Pontiac 455 its no bigger than a 350, thats one thing that's cool about Pontiac.

The new LSx engines all have the same outward dimensions, so the LS1,2,6,7 will all fit with similar mounting issues to tackle. Some folks are putting big cranks (4.0, 4.1+) and big bores in the latest iron and aluminum blocks, getting close to 500ci out of these Gen4 engines.