is this shift kit for our stock trans

http://www.3800performance.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=TEP-SK&Category_Code=T&Product_Count=1

is it for the thrasher transmission or our stock

That is for the 4T65e and 4T65HD. As long as you have a 97+ SC or 98+ NA that will fit your trans.

would i have to replace any gaskets

There is an accumulator gasket that comes in the kit. That will be replaced. The trans pan gasket is lifetime from the factory. As long as its in good shape and no one tossed it to put in a cheap rubber replacement, you can resuse it.

If the kit doesn't have one and you need a crappy rubber one, holler.....I've got a few laying around as emergency back ups.