Paul,
The LS3 is readily available as a brand-new crate engine from GM, they also sell a special wiring harness for engine swaps into older cars.
Link to an LS3 crate engine from Pace Performance, one of many sellers offering GM crate engines of all types:
http://paceperformance.com/index.asp...&ProdID=228371
The LS3 was recently tested, along with 7 other GM crate engines in the October '08 issue of Hot Rod Magazine. The stock 430 HP LS3 with the addition of a set of headers ran 11.333 at 114.23 MPH in a race-prepped 3334# '69 Chevelle, running 4.44 gears and a high-stall 8" race torque converter and 10.5X28" slicks.
I can see it powering my car to easy mid-12s, just like the new 'vette road tests show. The Tempest should weigh right about 3500# without driver with the much lighter all-aluminum engine. I expect to be getting mileage numbers in the mid-teens in the city and low to mid 20s on the highway with a 4-speed auto OD trans, the more aerodynamic '08 Corvette (around 3300#) is EPA rated 16/26 city/highway MPG.