TBSS exhaust manifolds are cheap, you can use some that Hooker puts out, just like them and flow the same.
Call a wrecking yard around you, ask for the price for a complete pull out, harness, accessories and computer ( need to get the pedal if it is drive by wire). Do not be scared of one with 100K miles, as long as you look at it and it doesnt look abused you will be fine. The LS motors will go past 300k miles easy.
In my neck of the woods, you can get a 5.3 like I said above for under $1K.
Oh and here is a supporting vendor with LSx engines for sale.
http://67-72chevytrucks.com/vboard/s...d.php?t=614503
"Our Ready-To-Run engine line all ship with the following:
-Brand new harness, not a reworked used harness.
-Full set of engine accessories.
-Pre-Flashed computer.
-Mass airflow sensor
-Drive-by-wire pedal and tac, if equipped.
-Our 1 YEAR warranty!"
you cant beat this price for everything you would need, except the motor mount plates and the advanced adapters trans adapter.
Good luck!