A way to do it will be to build a 60 deg V8 of 4.8 liters using the HF V6 architecture. You can build it on the same lines, they'll used the same pistons, rods, valvetrain parts, etc. A balance shaft can be used to smooth out the 60 deg configuration (ala Volvo's Yyamaha built 4.4 V8). It'll be a DOHC 32v engine making about 420~430 hp.
But, again, such a V8 will be a heck of a lot more work and money than a variant of the LFX V6 with different pistons and a cam grinds.
What about a 90 degree V8 from scratch, or by using blocks of the 2.0, 2.4 or 2.5 liter ecotecs? The Germans seem to come up with a new V8 every other month, Cadillac had the Northstar for 18 years and now nothing.
other than the LS's
what if GM made a seperate displacement for cadillac?
EDIT: acually if they could fuse those ecotechs tegethor it might not be that bad an idea