I think that I understand what you're trying to get at… I think that you made the example a little confusing by saying that you also get a smaller tractor (and you were intending this as boot received). Let's say you exchange tractors in a like-kind exchange; you also receive investment securities as boot, and you pay cash as boot. Now you're asking if the boot cancels out (net) in a situation (investment securities for cash)…
I believe the answer is yes, although I'm not absolutely certain. Generally the rule is that cash/property boot would net… liabilities assumed net… and boot received in the form of liabilities assumed by the other party does net. The only occasion that I see in which it wouldn't is if you receive cash or property as boot, and you give boot in the form of the assumption of a liability. If someone wants to correct me here, please do so, but if that is the case, the example below might help to illustrate.
EXAMPLE. John Smith exchanges property with an adjusted basis of $10,000 and a FMV of $14,000 in exchange for property from Mike Jones with a FMV of $17,000. This qualifies as a like-kind exchange. In addition, John Smith gives Mike Jones securities with a basis and FMV of $5,000 and Mike Jones gives John Smith cash of $5,000.
In this case, I *THINK* that the boot received (cash) and the boot given (securities) would cancel out. Hence you would have no gain to be recognized, and the property received by John Smith would then have a basis of $10,000.