Your first step is to get that barrel into the wood with the web between it and the rod hole an adequate thickness - ie: 5/32" at the breech and 3/16" at the muzzle. I'm assuming you have a blank that has adequate wood to do this method I'm about to recommend. The truth is, it would have been easier without the straight barrel inlet you already have.
Buy two lengths of 3/8" square x 48" (or longer than the longest barrel you may ever want) cold rolled mild steel. Grind a nice radius on the bottom of both to fit up close to the wood at the breech. May I assume that you have already sawed off the wood above the half way mark on the barrel channel?
Drill a series of holes into the steel as close to the edge without breaking out as you dare - mine are #23 drill. Start first hole 3 1/2" from your radius end, and you are making a left and a right - one for each side of the barrel. Holes 2 - 6 are 3" apart, and all the rest are 1 1/2" apart to the end of the rails. the close together ones are for the faster curve of the swamp.
Inlet the square breech end of the barrel so that the barrel rests on the top of the stock. Clamp the barrel at each end to the wood. Clamp the rails to the barrel while they lay on the wood. Make a centre punch to fit the holes and punch all the holes into the stock wood. Predrill all these holes to accept Gyprock screws - I like 'em (in spite of the ridiculous Phillip's head) because they are thin and tough. They only have to hold the rails to the wood so make sure they are close to a friction only fit - you don't want to break one off in your hard maple.
Screw the rails down to the wood.
Take off all the clamps and remove the barrel.
Grind or file the set off one side of a back saw, and cut down along the inside of the rails to the depth of the side flat.
Use a big gouge to remove the wood in the channel, at least down to the bottom of the side flat. Use a sharp wide flat chisel to cut STRAIGHT down along the inside of the rails to clean up that part of the barrel inlet.
Remove all the screws and the rails. Proceed to finish inletting the barrel until it bottoms - the waist bottoms in your straight inlet, the breech is level with the top of the stock, and the web is perfect.
Do 40 or 50 of these and you'll see why some of us grey beards use Dave Rase's service among others.
The bottom line - make some chips fly. Eat the elephant one bite at a time. And have fun.