Check these out: https://www.ovisonline.com/30-Heavy-...SABEgK2TPD_BwE
They are spendy, but the pair are rated for 450 lbs, you could always do two sets (total of four sliders) on the tray the machine mounts to if you want to be extra capable. And they can lock in place.
The furniture pads for the side... might still want to build a tray. In the event of any sudden stops you would want those machines secured down nicely so they don't shift, that's a lot of weight. Definitely do through-bolts to very large washers or full metal plates on the under carriage for any trays or mounts for the machines. Probably want welded steel trays and mounts too?
A bubble level and a good set of blocking to level the van out on, i figure a pinball machine wants a good level surface. I can't see any way to make the trays self leveling without alot of complexity. Would probably be simplest to level the whole van on its wheels.
Support you on having on hand the items or systems you know for sure
you will want. For me it was: ice chest, and second row seat and bed. Those i knew i would have and needed them to place in the space before i built out. There is no substitute for being in the space, and often the dimensions for items you get online are either wrong, or they communicate the measurements in a way that screws thing up.
So far my build process has been: 1)lay flooring, 2)build in the bed/dinette, 3)place the ice chest and second row seat in(not fully installed yet, but in place). 4)Then around that spacial understanding we use cardboard to template out our counters and over head storage. Those first three items we knew were uncompromising to have in the van and it is a good thing we did. Once i am past the point of having those things completely installed, then i will move to building the extra things in and buying addition items and systems. Hope that helps?
Are you wanting/planning on being able to run the machines on battery power, or will you always be around locations that you could run an extension cord to? If 9 times out of 10 you will have access to shore power, consider wiring the interior of the van with 110 outlets that you can plug into an extension cord. Then your battery system can be a back up system for the few times you don't have shore power.
this will be a sleeper/camper van as well? cool stuff, look forward to seeing what comes of it!