Hello Andy
All the M809 series trucks were normally aspirated. The NTC 400 will fit but you'll have quite the project on your hands to do it. The bell housing is different, you'll have to fabricate mounts, an oil pan, air intake stuff etc and change the oil pump pickup location. The NTC 400 will likely shorten the life of all the other driveline parts, too. The truck would still have the same top speed but you won't be doing as much downshifting on hills.
The M35A2 maneuvers off road and in close traffic a lot easier than the M813. Visibility in either is marginal. The M813 has much stiffer springs and even with it's longer wheelbase it can be a real bruiser on rough roads.
As far as driving them I don't worry about the speed. I figure I'm driving a 35-45 year tactical vehicle that was never intended to drive much over 45 MPH.
I strongly disagree with the statement that the M813 is a crude handling or clumsy truck. It is just plain bigger and has a bigger turning radius, wants to be on more solid ground due to the greater weight, etc. This is a function of it's size, not any defect. I like driving my M813A1 more than my deuces. It feels more like a truck. The deuce feels more like an over sized pickup truck by comparison.
The M813A1 has a much longer nose and the front bumper and winch seem a long way away. By comparison the deuce seems to be like driving a far smaller truck although the total length difference is likely less than 5 feet.
The M813, especially when loaded, can be painfully slow in hilly terrain.
Tools for am M813 really aren't much bigger. You may want a 3/4 ratchet wrench set, though.
As said, parts for an M809 series truck are bigger, cost more and cost more to ship. They both seem to need about the same level of preventive maintenance but the M813 will use a LOT more fluids. M813 air and fuel filters are a lot more expensive.
If you are new to MVs, I'd advise using a deuce as your training wheels. The learning curve will be steep enough without the CDL, weight, fuel consumption and other issues a 5 ton or bigger truck brings to the party.
Lance