From your title, I'm not sure if you are considering both box trucks and step vans, or if you're just mixing up names...as I was doing not so long ago
But I have yet to see a box truck with a stand-up pass through...assuming you're over 40" tall anyway. From a question I posted earlier, I've learned that box truck up-fitters have to pad off the main beam, raising box floor height over that of cab. But if you happen to see something that looks like a taller pass-thru, please do post a link.
Step vans have been added to my list of vehicles I'm considering and do searches on, and what I've found so far is this:
1) Step vans in newer condition or low mileage are far from cheap, holding value well above standard cargo vans. At least in south FL anyway.
2) Step vans that drop into the $10k-and-under range often have very high miles.
3) For reasons unknown, $10k-and-under vans have a "cockpit" that is usually beat to crap, instrumentation often looking so bad I'd want to replace everything there.
4) Having never tried to work on one I'm not sure, but it appears access to mechanicals may be even more challenging than a standard van doghouse.
But if the right vehicle/deal popped up, I'd certainly be happy with the conversion ability of a step-van.