I love it when guys come on here and say "I know it depends on condition but how much................."
You answered your own question. Can you eat off it or is all the steel swiss cheese and is there oil in the coolant?
As for swapping out the 707, who told you parts are hard to come by? You need to know where to look. And if you take care of it, like anything else mechanical, it will run forever. And unless it is an ex-FDNY C Model with high mileage, it is probably an atypical firetruck and has less than 30,000 miles on it- barely broken in. So the question remains- why would you swap it?
And, if you do insist on swapping it for an oil burner, drop a 237 Maxidyne and a 5 speed Maxitorque in there. You did remember to swap the rear axle, right?