Is this what i need to mount my ls motor to a 700r4 trans?
http://www.advanceadapters.com/products/pn-712500a/
Is this the best deal or is there something better for less?

 

That would work.
There are several adapters and flexplates out there, that kit has the weaker stock flexplate.

 

ok well i have a flexplate. Just no clue what its from. I know it was on a 5.3 not sure 2wd or 4wd.

Its part number 12580709 so if i can use it and just buy a spacer that would be way better imo.

anyone have an idea if i can use it?

 

Early 5.3's came with a spacer and a flexplate that would work. GM was basically adapting the older style transmissions to them in some applications.

 

any idea on the part number for the spacer?

Also this should have been off a newer 5.3 like 03+ it has the DBW throttle body.