[Clyde65]

You can usually get a 4.8/5.3 with everything ( harness,PCM and accessories) for under $1K here, with a trans ( 4l60e) for around $1500.00. That would be under 100K miles. Now I think you can do the swap for probably under $3K pretty easily ( 1500 motor\trans, 500 Fuel pump/pre-filter and misc, 500 harness rework and PCM flash, more misc $500.)

You will get better fuel mileage, depending on your foot, tune and cam you put on it, you will also get better starting and reliability. the LS motors will easily run 300K miles if you take care of them, my BIL had a 5.3 with 400K on it when he traded it.

Good luck with whatever option you decide is best for you!