It may be a silly question, but is "between hub centres" widthways or lengthways? I'm assuming it means between two lines drawn widthways across the vehicle that each pass through the centres of the driveshafts or wheel bearings (as they pass through the hub), and including everything vertically above and below those line line?
Yeah it's the 2 lines through the wheel centres front and rear (axles), everything in between. I guess you could also take the centre line of the hubs front and rear to create a 'box' although it's unlikely any part of the bodyshell is outwith those lines. I just take it as i'm not allowed to modify anything in front of the rear axle centre line and behind the front axle centre line. This includes the rear seat bulkhead and rear parcel shelf.
Interesting that a recently built Modprod car up here has had extensive bodyshell modifications within the wheelbase but has had loads of steel panels cut away and replaced with steel to change suspension pickup points. Very little of the original shell is left and it passed eligibility scrutineering. I think it's how the rule is interpreted again - Must remain in construction and material but modifying it is ok is how I read it. As long as you replace metal for metal and weld it as per original
A very grey area and it would probably depend on the scrutineer you get.
If you asked for clarity from the MSA, one scrut would say one thing and another would say differently. The rule isn't very specific about modifications.