The rivet fit for the underbelly, is indeed critical, but for a different reason.
The underbelly, because of it's thinness and lack of places to fasten it to, has a great tendency, while in motion, to flutter.
That being the case, loose fitting rivets will allow the underbelly to flutter, even more so, further artificially enlarging the holes in the metal. The underbelly also will shake when the running gear is not properly balanced.
That will result in premature failure of the rivet heads that were recently installed.
However, to fasten the underbelly, to almost a fail safe degree, use "large head 3/16 inch pop rivets." Use # 11 drill bit holes for them.
Costs a little more, but pays huge dividends.