I read today taht the test mule engines for the first viper was a 360cid V8 with two cylinders welded to the back, with a lamborghini casted block...
good read:
http://www.motoiq.com/MagazineArticles/ID/3740/Project-Viper-GTS-Part-1--Intro-and-History.aspx
from the article:
The Viper really is all about its engine. While Chrysler didn’t have a large budget for an exciting powertrain, they had a cast iron V-10 in the works for an upcoming Ram Heavy Duty. Lutz knew this big block was exactly what the Viper needed to differentiate itself in the sports car world, but it was going to be too heavy and it had to be made of aluminum for improved weight and performance. In the meantime, Roush Engineering took Chrysler’s old 360 cubic inch, 300hp cast ion 318 “LA” motor and grafted on two extra cylinders to make a 450ci V-10 for the test mule.
During this point in time Chrysler owned Lamborghini, who had a lot of experience designing and casting large aluminum engines and had a far more experienced supply base with much faster turnaround times and experience with a project like this compared to Chrysler’s slow to respond mass-production suppliers. Prior to the Viper, Lamborghini and Chrysler really didn’t communicate all that much, nor did they after.
Lamborghini completely changed the design of the cast iron V-10 to improve manufacturability and performance when cast out of aluminum. The heads and block share virtually nothing with the truck engine other than its 4.00” bore and 3.88” stroke. The first three V-10s were actually cast, machined, and assembled by Lamborghini in Modena, Italy before production started in Highland Park, Michigan. Once strapped to the dynos in Detroit, the first aluminum V-10 made an adequate 340 horsepower but had plenty of issues as well as potential. Meanwhile in 1990, Lee Iacocca tested the VM-02 prototype with the 360hp Roush-built cast iron V-10 and gave the project an official green light.