We currently live in an 1,500 sq ft one level ranch with a full basement under it. The furnace is an old natural gas 100,000 btu unit(which yes, is a hell of a lot of capacity for my house I realize). It has two 8"x24" cold air returns running to opposite ends of the house, that feed into a 24"x24" air box and pass through a 20x20 one inch filter.

Currently, one of the cold air return runs is fed by two wall returns, so each stud cavity is 3.5"x14.5"

The other end is fed by two floor returns that run through 9.25"x14.5" floor joist cavities.

Here is the problem, one of those two floor returns is located about two feet from a gas appliance in the kitchen, so it needs to move. Options are limited(for a variety of reasons), so I'd like to put it in the wall in another room, but that would obviously be less capacity(another 3.5"x14.4" wall cavity). We are in a rural area, and I've had both HVAC places that are available over to look at it. Both guys' best answer was "well it seems like you might be alright." Yeah...they don't seem to have any answers on how to figure out CAR requirements for my furnace.