I have not built one, so all of this is just in my head.
1. nice, strong lower frame with casters. Have something that drops down to take the load off the wheels, rather than a wheel lock.
2. Quick disconnect radiator/fan with long hoses.
3. battery and fuel tank mounted low (center of gravity), also quick disconnect.
4. gauge panel with ignition switch. You can source these from your extra parts or a JY run.
5. front mounts that clear both the pan and any headers.
6. fire extinguisher mount.
7. either a mid plate or a pair of bellhousings that will work with SBM and BBM.
8. Flex pipe to extend the exhaust. If going with headers, have a collector flange and the pipe. If going with manifolds, pick up some head pipes at the JY too.
9. Removable tool trays for both sides. Without fenders, you will want something to set that drill motor (prime) and tools on.