Well, lets see if I can answer some of these replies.
Varn - I am hoping to get an Ultraguage soon, and my tires are up to 50psi. The tires only have for 20,000 miles and are still looking real good, so getting new ones that are narrower and have less resistance will be a while and would cost approx 400 - 500 for a new set. The only way I can get 30+ mpg back and forth to work (primary use of car. 50 miles each way) is to drive 5 under the speed limit most of the way.
texanidiot25 - There were 4 cyl turbos used back in the late 80s and early 90s, but they weren't that great. The swap doesn't have to cost over 2grand. That is where getting an entirely wrecked vehicle comes in and using everything possible. As far as emissions, as long as the engine is from a 2001 or newer vehicle (must be to be legal) then I can use the items from both vehicles to make it compliant with little difficulty.
redneck - the main reason I have problems with gas mileage is pontiac did a final gearl ratio for the GTs at 3.29 where the GTPs had 2.93. My wifes 03 Impala has 3.05 and gets awesome gas mileage. I like the 3.8L, but cruising at 50 - 55 mph would be easier to feed 4 cyl instead of 6cyl. Not to mention it is solid cast iron except the accessory brackets. There is some serious weight to be shed up front.
Jim-Bob - It really doesn't have to cost that much. I have most of the tools needed to do custom fab, and what I don't have I plan on getting anyway. The hardest part will be figuring out wiring (accomplished by pinout diagrams and a software suite to rewrite some computer code) and the axles (not too difficult if measured correctly, and getting cheaper for high quality custom made ones). I toyed with the idea of getting a metro, but I am 6 feet tall and have had quite a few back injuries. The Grand Prix is really good on my back, especially with the heated seats.
D.O.G. - thanks for the heads up, It is something to consider. The main reason I got this idea was the new buicks. The new lacrosse and regal both have 2.4L 4cyl and are rated at 30 mpg highway. Granted they are using direction injection motors (can't afford those
) I did read there was a version of one of them with a turbo and a 6 speed manual tranny. Both of those buicks weigh more than my GP.
NHRABill - As long as I use a engine/tranny from a car at least 2001 or newer. It has to be able to pass an inpsection and emissions, if it is required in that state. As far as recouping cost, the primary reason is to save money on fuel. NO matter what I do, the 6cy will never be as efficient as a 4cy. The other part is like you said, to be able to say I could do it. I find it amussing you talk about reinventing the wheel. The GP is a mid size sedan. One of the few that did not offer a 4 cyl base model. If you look at current manufacturing trends, most of them are using 4cyl and turbo 4cyl in their mid size sedans.
I think this would be an interesting project. Of course I would have to do some serious shopping around to find the right engine/tranny combo at a decent price. There will be some fab work required for motor mounts, exhaust, and custom axles. The other upside is it would reduce weight up front. That would help balance and handling along with lowering the overall vehicle weight. AT 3500lbs, the GP is a hefty car. I appreciate the comments, keep them coming it helps to make sure I am thinking along all avenues.
J