Based on the well tanks on the Furness Railway, plus sight of drawings for other engines of this wheel arrangement, I can offer the following.
The rear carrying wheels are only carried in the outside frames - this keeps the springs and their mounts away from the firebox, keeping the wheelbase down.
The front carrying wheels are (usually) also carried only in the outside frames, for the same reason with respect to the cylinder block.
It is important to keep these carrying axles as close as possible to the firebox and cylinder block to keep stresses down on the frames.
The middle, driving, wheels are supported by both the inside and outside frames, with axleboxes either side of each wheel. Early GWR broad gauge engines also had a fifth bearing, in the middle of the axle. (There was room!)
The inner frames ran above the outer axles, if they reached that far.
Now the fun bit... Where did the inner frames begin and end? Think of them as being the braces to the belt provided by the outside frames. These latter are the main structural component of the locomotive, as everything else is fixed relative to them. But the role of the inner frames is to support the power unit, i.e. the cylinders and the crank axle, plus valves and eccentrics. As such, the inner frames have to begin by being bolted/riveted to the cylinder block, and must go beyond the driven axle. Since they will need joining together, (or keeping apart!) then a spacer of some form will be needed beyond the reach of the cranks and connecting rod big end, in addition to that provided by the motion bracket (which is also quite possibly supporting the mid length of the boiler). This will have to go in front of the fire box.
On the drawing you have, the bottom of the inner frame is quite clear between the leading and driving axles, but there is no indication of this besides the firebox. It is likely that both the inside and the outside frames supported the buffer planks at the front, but only the outside frames at the rear: it is also likely that there was no footplate in front of the cylinder blocks.
The outside frames may well have been a sandwich with thin plates either side of a substantial thickness of wood, necessary due to the fact that the high quality steels which rendered this unnecessary lay some years in the future: the Furness engines had outside frames composed of 5/16” plates sandwiching timber of 3 ⅜”, or 4 in total. You could easily cut the outside metal frame from 5 thou sheet and emboss it to represent rivets, and sweat it onto a piece of 1mm or so metal.
How much of this you actually follow is up to you. It is perfectly possible - and a lot easier - to build a working inside framed chassis, and to make the outside frames cosmetic. I know I would, and having done it once the same way as the prototype, so would the best modeller I know!