Dr Red,
The bolster on your lock is what determines the thickness of your lock panel, which should be duplicated on the side plate side. The swamped barrel is what determines the shape of that area, thus giving the whole gun that nice trim, tapered look.
I have no idea why he would want the side plate side NOT be parallel to the swamp in the barrel. That is what gives that nice tapered appearance. To me, it would flatten out that side of your gun, no longer symmetrical, and make it look quite unsightly.
When building a kit, quite often, I have almost eliminated the original pre-carve side plate molding in the process of making the same thickness as the lock side. There is usually plenty of wood that needs to be taken off.
I would just continue to inlet your sideplate where it is, and with a large sanding block, sand down the side plate side till it's the same as the lock side. You'll love how much it slims the whole gun down.
Hope this helps