[Number 3]

From Autoblog:

Back in August when General Motors introduced the all-new GT2 class Corvette C6.R, it ran downsized 6.0-liter version of the 7.0-liter V8 from the long-dominant GT1 car. At the launch, Corvette Racing program manager Doug Fehan told us the 6.0-liter was just an interim engine. With revised GT rules on tap for 2010, GM was already planning a brand-new engine for its race Vette.

Unlike the 6.0/7.0, which is a ground-up race engine that only shares basic architectural dimensions with the production small block, the 2010 C6.R's V8 is a new 5.5-liter unit that will indeed be derived from the production engine found in roadgoing Corvettes. In fact, the 5.5-liter race engine will be built at GM's Performance Build Center alongside ZR1 and Z06 V8s.

I actually tried looking for the 2010 Rules in ALMS, but my guess based on this is that there is a displacement rule and a "production based" rule. Mostly because other than marketing ("our race engines are built on the same line as the Z06 and ZR1s") why would you leave Katech who has built the Corvette Race Teams engines for some time now. But I am only speculating.