The newer transmissions are a little better, but the 4 speed ones of that vintage are not bad by any stretch. The main improvements are in the computer control and programming; the actual cruising gear ratios are identical (or close enough to identical for any practical purpose).
I'd have no concerns about the V10 engine. It should be adequately powerful for a rig of that size. You aren't going to win races against Corvettes, of course...and maybe not Chevettes...but that's no surprise. I have a 32 foot class C on a '98 E-Super Duty (which was renamed to become the E-450) with the V10, and I have no complaints about the chassis or its performance.