No offense intended - why?
I have to ask "why" too. Replacing the air variables with vacuum variables is a construction nightmare! I put vacuum variables in my homebrew linear so I know whereof I speak!
First of all, brackets have to be fabricated to hold the caps in the proper plane. This means drilling mounting holes in the chassis with the possibility of interference from other components under the chassis.
Second, a strap connector must be fabricated for the rear terminal of the cap and flat copper strap has to be rerouted because it isn't likely the original wiring will reach.
The caps are multiturn and require a turns counter which cost about $50.00 each.
Mounting the turns counters to the panel and aligning it with the cap shaft is a major undertaking itself.
The turns counter shaft and the cap shaft will no doubt be of different diameters so a coupling for each will have to be fabricated.
There's a 50-50 chance the turns counter will change numbers opposite the increase/decrease in capacitance. (Since I couldn't remember this on occasion I had to fabricate a special tag to remind me)
After all of this what have you accomplished other than ruin the panel of a fine linear? Zip!!
However, if you really want to do this, good luck. No doubt a phone call to the factory will get you the cap size info. (I can't imagine that this information isn't on either the parts list or the schematic)