wow... that side-by-side video at the end is a complete crock of ----. Maybe if the guy still thought the swap was a good idea after he finished, he would use video of his actual car.
The speedo isnt going crazy fast but it's still faster than a 2.5/604 with intake and exhaust would be. I really dont think that porting and polishing the heads on those things would do jack ---- if you ran a stock plenum and TB.. hes using the stock plenum off a cloud car 2.5 in one of the motor pics, but the finished install shows the stock avenger/sebring plenum installed. Even if you could move the limiter on those computers (how did he do it? B&G maybe.....) the thing wouldnt rev out on stock plenum and cams for crap. On a stock motor it's dying off at 5500 even though those heads on a 2.5 could go WAY higher.. it would not accelerate from 7-9k rpm like that. And his tach must be full of crap because a 2500 stall on a torqueless motor is not going to actually stall at ~4k like the video seems to suggest, and i dont think the stock 24v valvetrain could do 9k, and i dont know if you could spin a 604 to 9k without big converter problems. Also, i know of every possible 604 gear ratio mashup that doesnt involve custom gears, and there is no gearing you can stick in the 604 that gets you 9k rpm @37 mph in first gear. even with the lowest possible ~4.28:1 final drive, you would still be doing 59mph @9k rpm in 1st gear on a tire diameter like what he shows in his vid, and the tire you would need to make the math work would be 19 or 20" in diameter.. which only exists in racing tires for 13" wheels which wont fit over 2g dsm brakes.
So, theres a lot wrong with that clip at the end.
But yeah, it's easy to do a v6 swap into a 2g dsm. You would definitely have to run the mitsu-style intake manifold though, and probably dist cap and wires too.