Re: Converting '04 limited to SRT-6
The easiest way to do it would be to get a "complete" engine and "tranny drop" out from a salvage company like "LKQ" and make sure the the SRT's wireing harness and all the computers come with it.
Everything should bolt right up. Probably cost at least $5k in "salvaged parts) to do. A lot easier just to buy an SRT in the first place. But since you didn't, and you'd lose too much money trying to sell the 04 that you have now. maybe this might work for ya.
I have know idea what a "reputable" garage would charge you to do the swap. Let your fingers do the walking, and search the yellow pages.