For the whole package I think Callaway is the most expensive. For the whole Supernatural package it was about $65K. They give the option of boring the block out to 383ci. If I remember correctly the engine work costs about 15K.
LPE has a 383 stroker kit for the LT4/LT1 too. I think it's about the same as Callaway, but as Hutch mentioned, they have a better rep when it comes to engine work. I dont remember if DRM has a package for the C4's.
There are a whole lot more tuners for 'Vettes, but those three are the ones that pop into my mind right away.