Too much work and money? How so?
The guides are going to follow the reamer? How are they going to guide the reamer?
How are you soldering the drive bar to the reamer? Butt joint? Sleeve?
7/8-20? 7/8-18 is a standard. But if you have the taps and matching die already, it doesn't matter.
The thicker tube will be safer. Less stress will be transferred to the original barrels..
Are you going to fit the breechplugs first, and then set the liners hard against them, or sleeve the barrels and then fit the plugs?
How are you making your plugs without a lathe? The plugs have to fill the remains of the chambers completely. Unless you cut most of the breechend of the barrels off, the plugs are not the same as typical plugs.
Please do a photo tutorial on lining your barrels. Your experience will be valuable to others.
Go to gunpartscorp.com, and have a look at item 584990. Might be a better start than what you are dooing.