Just one experience.
There is a very strong possibility that the angle of the flange on your muffler is not at all at the same angle as the flange on your manifold.
This was a very common problem with many later aftermarket Model A mufflers.
Also, the "vertical" angle of the muffler has to be positioned, (both ways), such that it aligns with the "vertical" part of the manifold, to insure that the two (2) meeting flanges are almost perfectly concentrically aligned.
In the past, I have had success with lightly supporting the rear of the muffler, then adjusting the rear of tail pipe until it "appeared" to almost be perfectly "vertically" aligned, (both ways), with the vertical part of the exhaust manifold.
Then tighten the clamp; and lastly, install & tighten the rear muffler tail pipe support.
If you ever wonder "if" this joint ever worked, I now have an Aries SS muffler with a new matching exhaust & intake manifold installed on the block with gland rings, & this joint is very well sealed with no whistling noises & no odors.
One can immediately notice that Aries, being far more careful & concerned, insured that they have the correct muffler angle & flange angle.