Have you measured the old truck yet?
How wide outside of front tire sidewalls now, and also width outside of front fenders.
How long is engine compartment from firewall to radiator? And or what engine is in it originally?
What size tires on it now and best guess at how tall might still fit fine?
The buss pictured likely has a CAT C7 which is inline 6 and from all I hear not very desirable, expensive and troublesome, or can be later. Or it may have a 3208 which is a V8, slightly better reputation but not by a lot, probably to wide to fit in engine compartment.
Start with measuring the truck and your best guess at what modern engine, front axle width, and wheels might fit, then hunt for chassis with those.
