Truck Stacks question

Wow. You've had them awhile. Must have not needed them right away.
I included most of these tips in your instructions.

Anyway, they are test fitted here in a try-fit stack. However, stacks can vary between manufacturers, even though the industry standard is tubing is sold designated by OD, and truck stacks are almost always 0.065" steel, so you should already be close to fitting. However they are sometimes sloppy on the burrs they leave on the edge of the cut. Especially with miters, and especially with offshore makers - usually China. Those are cheaper (mine are all American) but their QC is wanting - aka nonexistent. If you feel a burr on the edge of the stack cut, use a die grinder, (Harbor Freight has them cheap), get a carbide burr (also at HF) and carefully grind down the burr on the edge of the stack cut. Of course you need a compressor for the die grinder. If you don't have that, an electric drill will also work, albeit slower. A wired one will have more power and not run out of juice. Another soln is, use lamp black (or spray paint) on the silencer seats, insert them while twisting a little, note where the soot (or paint) is scratched off, and use a grinder flap "blender" wheel in a 4 1/2" angle grinder (both also at HF) to reduce the metal a little. Try the fit. Rinse and repeat. I actually use high-grade mostly wired power tools (Milwaukee, Dewalt) as I exceed their duty cycles and use them a LOT. But for non-production use, you don't need the expensive tools.

Another point is, the stacks are not truly round. Neither are the silencers, They are both slightly oval. So they will fit one way better than the other. Note there are 2 screw holes in the silencers so you can check the fit in 2 positions, and you can try the silencers in the left or right stack to find the closest fit.
I actually chuck the silencers in a lathe and use an angle grinder with flap-blender sanding wheels to reduce them some while they are spinning - which I did with yours, prior to the try-fit. That's considerably faster than the above tips.

Edit:
I should mention, I also fab'd an internal-expanding hydraulic press to push outward on stacks from the inside to true up the roundness for stacks that are malformed more than usual. Of course this is a dedicated tool really only viable for a business that needs it regularly. A similar concept could be done by squeezing from the outside, with great care not to mar the chrome. Like with a shop press (with appropriate press plates) or a large bench vise. Of course, heavily padded with say leather layers and microfiber rags.