A few suggestions
(as always YMMV)
As you have your wheels cut, build them up, install and drive with them installed in a bunch of situations and make sure everything is hunky-dory
(translated to queen's English - working properly
)
You don't want to in the future after you have changed the wheelbase, changed suspension, tweeted, and modified have a tracking, wobbling, or vibration issue and wonder if it is the tires/wheels or one of the other modifications.
If you lengthen the wheelbase, as long as your rear spring mounts can be reattached to the original frame, it is very straightforward. Grind, drill the mounting rivets (remembering they are rivets so even with the heads removed they are still very tight in the mounting holes), measure 5 times, and install in the new location using graded bolts.
If you need to extend your frame for the back of your box, no big deal if it is not mounting any of the suspension. Just mount with bolted doublers and you would be good as it will only support a small amount of the load from the box. BOLT to your frame, do not weld. Yes, I know your shortening cuts were welded, but in general, frames are heat treated. The Japanese firetruck modification company certainly did a bunch of these conversions and had engineers that graduated from the Tokyo Insitute of Mechanical Engineering. I'm guessing you don't.
No need for fancy 3 or 4-point mounting subframes. 'Frame on frame' is just fine as your box, being an ambulance box is super ridged, and being a heavy truck and your 'home' you are not going to be rock crawling, etc. Plus you want your box as low as possible, something a subframe would negate.
Stick with leaf springs. While coil or air suspension sounds cool, and in most cases would give you a better ride, that is not happening with your setup. Left springs are straight forward and there is a reason all big trucks (with the exception of Unimogs that are offroad tractors masquerading as on-road trucks
) use leaf springs. Nothing is going to give you a plush ride, especially being a cabover, so keep it simple. Down the road, after everything is built, installed, and tested, if you want some improvement have left springs (maybe even parabolic leaf springs) build for your weight, weight distribution, and driving conditions.
One thing that will improve the 'ride' is an air-suspended seat(s). Unfortunately on cab overs usually there is very little room for a full travel air-suspended seat. Normally just a few inches, so a very limited selection (a search here on the Expo Portal for 'Fuso suspension seat' will give you an idea), or if you are lucky perhaps some pull-outs from a Unimog. The single best ride improvement on any of my big trucks was a full-size (10+ inches of travel) 20+ way adjustable air ride seat on my Kodiak. I tried a mechanical suspension seat (modified from a Mitsubishi Montario) on my Fuso, but just not enough room for any meaningful use without slamming my head into the roof. My friend has suspension seats from a Unimog installed in his MB1017, and they work OK, but he is short so he is not headroom limited.
