Marc: this and now is your opportunity to work on the slenderness of the forearm. The web wood between your barrel and the rod hole needs to be only about 1/8", or just enough wood to allow for the barrel tenons to not interfere with the rod hole. And that's all. If your pins are 1/16", you will need to pass your drill so it just touches the bottom flat of the barrel, and goes through the tenon. You only need 1/16" of steel below the hole in the tenon to support the pin. That will give you a tenon of only 1/8".
So once your tenons are pinned to the stock, cut your rod groove in the upper forearm so that you will have a web of 1/8" or so. 3/16" is a lot and more is a mistake. Once your rod groove is cut and oriented so that the drill will run true, drill your hole.
For a .40 cal. a hole of 5/16" is great. Your hole should end up 1/8" from the barrel and even with the breech plug. Now measure down and mark the side of your stock where the bottom of the rod hole is...the advice given above describes how to determine this accurately. Having drawn in the rod hole on the side of the wood, lock side, give yourself 1/16" - 1/8" for the wood under the hole, and draw that line onto the stock too. Take away all of the wood that is below this last line. I think you'll have to re-inlet your trigger plate, if you have done that already, because you have much too much wood along the bottom of your stock.
So, to reiterate: on the side of the wood, lock side, draw a line indicating the bottom flat of the barrel. Next another line indicating the bottom ;line of the web, 1/8" from the bottom of the barrel. then the next line is the rod hole's bottom line using 5/16" as our example. Finally, the bottom line is the underside of the stock.
This is a rifle I built using those guidelines, and I got them from dimensions acquired by Tom Curran and Eric Von Auschwege in the Metropolitan Museum of Art in New York. My rifle inspired by those dimensions, uses a swamped barrel 1' at the breech and .40 calibre, and the same lock you are using. The rifle is alive like a bird in your hand.
I agree, your Movavian build will be more voluptuous.