Well, if it was me, I'd just save up for a supercharged motor. On your car I think it would be worth it.. Its a '99, fully loaded, all decked out the way you want it already, already has a 4T65E, etc... Swapping in a L67 should be a snap! Alot easier than doing it in our 95'* anyways :( Thats just my .02 but I think once you get all that stuff for your N/A motor, and get it all going, your gonna test drive a friends GTP or SSEi or something, and wish yours was supercharged. :(
Save up for the */c motor, buy it, then build it up before you even put it in the car. Cam, Nose Drive, Pulley, etc. etc. etc. would all be a BREEZE if the motor wasnt in the car yet.
One thing I would suggest though... Dont just go buy a motor, your gonna need more than that. See if you can buy an entire front clip.. I'm talking L67 motor, 4T65E-HD Tranny, Wiring Harnesses, Computer, Halve-Shafts, Etc.... I've gotten prices on complete front clips and they are roughly 2500, so youd def be better off.
Only other thing you might want to consider, is stripping your car back down to stock, selling it on e-bay since its still pretty new and worth some coin still, and finding a nice black identical '99 SSEi to replace it. You might have to fork over a couple thousand for the difference but it would be well worth it and no headaches of a swap.
Just my suggestions for ya, it all comes down to how involved you want to get
2004 Impulse Blue Metallic GTO
Black Powdercoated OE Wheels
Predator w/ Custom RWTD Tune
Hardwired V1 w/ Remote Display
HoŽn Highs, Lows, Fogs
B&M Rippper Shifter
GTPprix Shift Light/Buzzer Mod
Split Rear Exhaust w/ Custom Tips
1995 Bonneville SSE - Sold!