having been thru this, lately if I were looking for a complete engine/ trans deal I think I'd go the L76 route w/ 6l80
same as mentioned earlier comes in G8 cars and Caprice.
here's one on ebay for not a bad price really...there's several
https://www.ebay.com/itm/2012-Capric...UAAOSweHtdk4Zv
The reason I say this:
aluminum block 4.00" bore
rectangle port heads, lots of potential
car intake so it's low profile
exhaust manifolds look like the 2010+ camaro pieces, they should fit our trucks with no cutting needed.
6 speed auto...you can run more rear gear, still get great cruise mpg
they do come with the oil cooler which I would delete...