The chording shown above for the last two lines of the chorus appears to be based on Arlo's version (which I think may actually have been closer to Eflat, F, G rather than Bflat, Am, G, although I havent listened closely to it for a while).

I think Steve Goodman's original (in C) was simpler and fit the slightly different the way he sang that part of the melody - he just had it go:

I'll be [Am]gone five [F]hundred [G]miles
when the day is [C]done.