LQ9 is a nice engine. It has slightly higher compression, heavier duty rods and floating piston pins. Good engine for sure, I had a couple of them. LS3 will bolt up, you will different rocker arms for some of the valves. I would try it as-is with a cam and spring change. It may give you the performance you seek w/o buying heads.
*69 SS 502 EFI /224* HR/9.8:1/T56/3.90 11.5/124mph
*11 C6, 6.2, 6spd
*96 Stang 6.0 turbo 10.5/135mph
*61" Scag Kohler 22hp 7mph
***In progress...04 Mustang, 454, TKO***