Couple of things.

Yes you might be able to fit the barrels such. However the accepted way is to make them slightly protruding from the monoblock face and to smoke and file/hone them in for a perfect fit and then adjust the rim cut with the reamer to get perfect cartridge headspace. If the rim cut is slightly too deep( and we are talking 5 thou or so then the cartridge base can get a running start at the action face. ( I will ignore cartridges clinging to chamber walls as it complicates the issue and we have to start looking at chamber polish etc)Much too deep and you might get erratic ignition as firing pin force is used up shoving the cartridge forward.
Your calcs and drawings would have to be exactly spot on first time with no possibility of correction. That is hard.
In the overall cost of this sort of project then the cost of a reamer is insignificant. I use Dave Kiff. @ Pacific tool and Gauge (541)826-5808. My .450#2 was $ 147.00. IMHO it would make your life a whole lot easier. Also whoever you get to do the chambering may not have the reamer in the first place and they would proberbly add it to the cost of the chambering and threading. You could buy one and supply it to them to be returned with the barrels.

As to pressures etc. Your 16 guage is proved at 32,000 psi. But that is proof load, not the service load. 1 1/4 oz 16 guage shells are about 11,000 psi that is the service load for that firearm.

Hoop stress will not be the issue. Breach thrust will.

Take the area of the base of a 16G and multiply by the 11,000 that will give an aproximaton of base thrust. so .430 x 11,000 = 4728 lbs
Do the same with the cartridge you plan to use. ie 450 3 1/4 NE av. 34,000psi = .233 x 34,000 = 7927 lbs
16 g at proof pressure is .430 x 32,000 = 13,760lbs

So the 450 has 70% more breach thrust than the service load and 60% of the total at proof.

you are using up a lot of the safety margin.
This does not mean that it wont work but it should be bourne in mind. This was why i used the .450#2 as the ave pressure is 26,000 psi. For the same balistics.
My rifle was re-proved at 52,500 psi.

Hope this helps

Regards