Hey, I'm not throwing stones, I'm just add odds as to where it says you can modify a stock tank in any way. I know certain cars have issues with pickup, and starve, and must run 1/2 or 3/4 tanks to avoid that. Since gas has weight, carrying extra can certainly be a hindrance to acheiving minimum wieght, and many would see the extra weight as a competitive disadvantage.

If the tank has internal filters, and access, I suppose substituting a similar filter that has increased slosh resistance could be considered legal, by many.
But adding componentry in the tank that is non stock is verboten based on the IIDSYCYC foundational language in the ITCS/GCR. Now, you might find that dumb, but maybe the rules makers had reasons for it. Perhaps they felt that they didn't want folks mucking around with stock tanks, and that they preferred mods to be done within fuel cells, which are designed for such things from the outset. I don't know, I wasn't there when that rule was written. A letter to the ITAC requesting a rules allowance to add internal surge tanks to stock tanks would yield a response though, I bet.