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Scooter hit it on the head. The drawing of the mount that he posted is close to the Street & Performance mounts and retain the engine in the stock position in the car. The mounts that he has bolted up to the car are a little different and move the engine and trans forward ~1.5-2". This allows the shifter to line up a little better for a 6 speed, you won't have to cut the trans tunnel and also helps with the heater outlets from the water pump. I am using mounts of the same design as the ones on Scooter's car that move the engine forward. They are pretty easy to make and I have had no problems for the last 2 years.
Between my site (whenever it's done), Jeff's, Budman's, and GameshowBob's, one should be able to find almost everything that they would need to know to put an LSx motor into a C3.
I've got templates if you need them. Most are pretty easy to duplicate and modify as needed. The bigger issues will be the electronics and the expense of all the little things. I've probably spent $1000 on just little stuff, like connectors, hoses, and intake parts.