Fitting a V8 wouldn't probably be that much of an issue, but fitting a Hemi is likely more the problelm. The heads are stupid wide and the exhaust setup on them is typically a little different than wedge heads. If you look at most heads, they actually taper from the block mating surface up to the valve cover surface, so the exhaust actually points more sideways when installed on a V shaped block. On the hemi, the heads basically square (if not tapered out a little from bottom to top), so the exhaust actually points out and down and right into the frame rails of lots of cars. I saw them on my 67 Dart hemi swap. You don't install headers bolts from the side, you practically install them vertically from the bottom of the car.
Seeing as the V10 is sort of based on the old LA architecture, a V8 would likely fit, but who wants a 5.2/5.9 Magnum engine in a Viper? Dodge had this same issue back with the 340. The 360 came to be known as a bit of a pig when it came out because of EPA restrictions cutting power so bad, but what a lot of people didn't realize is that the 360 was basically a 340 to start with. The 340 had such a reputation for performance that Dodge didn't want to release such a detuned version of it, so they rebadged it as a 360. To this day people still think the 360 is crap (partially just because aftermarket parts aren't as available for it), but it can be made into one mean 408 stroker.
What I'm mostly getting at is that the Hemi is the Dodge performance engine namesake. If the Viper gets a V8 that's not a Hemi, I think most would think it was an inferior engine for one reason or another.
