To clarify what was mentioned before, the NEC can consider a generator as a separately derived source, but does not always consider it one. It depends on the type of transfer switch used. Most consumer transfer switches do not transfer the neutral and therefore do not count under the separately derived source requirement. The installation of generators in a way that is code compliant with the NEC is complicated with lots of room confusion, it is best to ask your local inspector how he wants it done. As to the point about frame grounding there is some gray area here in the NEC as it falls back onto the installation manual for the generator for exception to the general rules, and since these military surplus generators have no "installation manual" you tend to be caught in a bit of a no mans land. There may also be state regulations that are not part of the NEC regarding generator installation, Florida in particular has been getting very picky about this the last few years, often requiring engineers to sign off on concrete mounting pads, as well as limiting placement near structures or property lines, etc.
Ike