Rob,
The Ultimas and Moslers both have the advantages of very light weight (2200lbs), and good aerodynamics. They are a bit closer to kit or race cars than the Vipers are, and therefore are spartan, but very fast.
The engines in the Ultimas are usually Chevy V8s, so your buddy could drop in a nice LS6 motor with 425HP and be competitive with the stock SRT-10 in performance, or he could throw in a Lingenfelter twin turbo 427ci V8 and be VERY hard to match.
The trick is even though he has quite an advantage in weight, the important part is putting the power to the ground, and providing balanced performance.
Also the outcome depends on if you are shooting for stoplight-to-stoplight racing, 1/4 mile runs, or full track competition.
Both cars are really built for track racing, and if you were shooting for just a 1/4 mile competition you could easily build an 8 second capable Mustang for what you will most likely spend on upgrading your future SRT-10, but of course the drag car would be functionally useless for anything else.
Along the same lines, you might want to look beyond the competition against you partner/friend to owning and driving the car later on. The Ultima may or may not end up being a faster car, but it is not going to be a car that you would feel comfortable taking to the corner store, or for a 400 mile trip, let alone commuting. Whereas a Levin modded Viper will have more power than you could likely imagine, or most likely use on the street (safely), but also be relatively comfortable and reliable for regular driving.
I think that with a big budget, Doug Levin will be able to make the SRT-10 as fast as it can be. The biggest trick will be your drivers training to learn how to utilize all of that power.
It's no slight on you, but even a stock Viper can be beaten by a Toyota Camry if he just sits and spins his tires forever.
Whatever you do, give us details on both of these dream cars because outside of automotive magazines, it rare to see "best of the best" car competitions, and it's a real treat for us car enthusiasts.
-Dean.