Probably the most economic way to do the swap is with a truck engine (iron block). As stated, the LM7 5.3 is a good candidate. They are plentiful so they are cheap. One thing to watch out for is if the engine is GEN III like an LM7 (5.3) or an LQ4 (6.0) or if it is a GEN IV like an LMG (5.3) or an L96 (6.0). The GEN IV motors have a different reluctor ring on the crank than the GEN III motors. The GEN IV motors have drive by wire throttle bodies. If you want a drive by cable throttle body, then stick with a GEN III motor. Even late GEN III motors are drive by wire. I think 2003 1/2 ton trucks are drive by wire, for example.
If you can find one, a takeout engine with all the accessories, the PCM, wiring harness and transmission, that is likely the most economic route. If you go with a GEN IV engine, you will need to get the accelerator pedal as well from the donor vehicle. An LQ4/LQ9 and 4L80E would be the ideal find. That combo is sought after so it will cost more than an LM7/4L65E. The 5.3 with a cam, headers and tune makes plenty of power. I have that setup in my daily driven 2008 Sierra.
The truck front accessory drive, oil pan and intake manifold can be problematic with fitting our chassis depending on what mounts are used. There is a company making brackets to relocate the truck accessories to package them to fit the car chassis better. This is likely the best route to go with for your front accessory drive. Most people just swap the truck oil pan because it hangs so low. The GM "muscle car" oil pan (Hummer H5 pan) is a popular choice. Holley makes an entire swap system for the G Body with accessory brackets, mounts, headers, trans crossmember and oil pan. It is a really nice engineered solution but it is not cheap. Depending on what mounts you use, the truck intake manifold can be problematic on hood clearance but there are plenty of people running them in these cars. The truck intake is ugly in my opinion but its long runners make good low end torque.
Are you good with an automatic transmission or do you want a stick? The automatic will be easier and less expensive but everyone would understand if you wanted a stick. Hope this helps.
