Hey Pete, I'll chime in on this if you will. I have had a LH GPR flintlock in .54 with the 32" barrel. I have built 2 of the Haines rifles with the 38" barrels. I can tell you for certain it will feel lighter, it will balance better, and you can adjust that powder charge to fit your recoil level.
I would consider carrying a shooting stick or a bipod, I also use mine as a walking stick to help an old man with shot joints traverse bad terrain.
This is the type rifle you wanted several years ago.
The biggest negative I see is the cast off in the stock. I, being a lefty have never handled a rifle with cast off for a right hander so I don't know how it will feel or how well I could shoot it. But consider this, the stock could be bent back straight by a good stock man like JerryWH.
Also be aware, fitting a rear peep to a swamped barrel is not as simple as it seems, that's why most are hand made.
I hope you can find a way to make Dennis's rifle work for you.