You will not find a 34 or 36' foot coach with a 450hp Cummins engine. The chassis builders only install engines according to weight. The biggest you will find for those length and weight are 350hp.
Some of these guys suggest getting big 500hp engines like what they have on their coach. The only thing they do not tell you is that the coach they have is 45' foot long and it takes every bit of that power to move that much coach.
The only engine that I would not get is the 5.9 Cummins engine. Those engines are under powered in my opinion, but the ISC is actually a big block engine which would be good, and on the Cat the 3126-E all electronic engine has lots of power because it did not have all the smog devices added later on to the 350hp Cat engines. But they are very reliable engines. Then Cat made more powerful engines also just like Cummins. They had a C-9 at 1200lbs of torque and all the way to their big boy the C-15, but you will not find those big engines on a small coach like I mentioned earlier on my post. You would have to get a Newell to get the C-15.