The following is IF the 235 you have is 54 or later, if it's the old High Torque/splash oiling 235 none of the major parts will interchange.
The 261 will bolt in your older truck as follows:
The head will work fine, just compare to the 261 head and drill tiny steam holes between cylinders on the deck surface, use the 261 head gasket as a guide.
You want to compare the front mount plate behind timing gears, and use the original one if you need to for the correct front motor mount.
The only real snag will be the long water pump pulley of the 58, will hit your radiator. As you will have the 261 out of the bus, I'd install an adaptor plate which will allow you to use the older style/original short water pump. I believe the maker of these plates is a bolter on this site, or check eBay for these.
Other than those couple items, this is as close to a direct bolt in as you will every see.
