Well, we gave you some real good advice on the SASS Wire.

But, why don't you just throw a load of Trail Boss like you want to load, into a dead-primer case. Then hold one of your slugs up next to that case with the case mouth level with where you want to crimp the slug. If the powder level is higher than where the bottom of the slug is, then, yes you would have some compression. Once you do that you will know for certain! And you will have done it your own self. I could do it but I don't own (have never bought) Trail Boss, and I don't have one of your bullets to test with!
1. I REALLY doubt you need to drop to 1.550 overall length to get ammo to run in your gun. My Pioneer short stroked 73 runs any ammo that fits in the carrier shaft - right up to 1.600 OAL. A couple three dummy rounds made with just case and slug would tell you REAL quick (less than 5 minutes work and testing), then you can knock out the slug with a bullet puller and reload case like normal.
2. Trail Boss, like most other smokeless powders, can be compressed a little bit without raising pressures dangerously. Unless you are trying to load at the VERY TOP END of MAX LOADS, a 20 thousandths deeper seating depth of the bullet is not going to blow up a gun. You sure don't need to have anything like a MAX 45 Colt load to shoot either cowboy or Wild Bunch! So stay at a moderate load that makes about 165-185 power factor in your rifle and you have PLENTY of Trail Boss for our games.
3. And if you ARE trying to load a hot load (above SAAMI pressure levels) don't do it in a 73 that has an expensive short stroke kit in it. Get a 92 or even a clunky 94 that was (somewhat) made by the factory to handle 45 Colt cartridges. THOSE guns will handle pressures up well over what a 73 will take.
From your line of questions, I would suppose you haven't done a lot of reloading, especially with cast bullets to make low velocity loads. Can I suggest you get and read the Lyman Cast Bullet Handbook? It will tell you info you didn't even know you needed to know.
Good luck, GJ