If your pipes fit together that badly (that you feel you need a sealant), why not just expand the smaller one till they're a tighter fit?
The cheap plain steel clamps have their place, but if you want to get it apart again later, with less effort and destruction, avoid the goop and just put a band clamp on tight fitting pipes. Make it stainless if you anticipate some portions of the exhaust wearing out faster than others, for example using a stainless muffler with an aluminized pipe after it instead of stainless, so there is a greater possibility you will need to remove that clamp instead of just detaching or cutting out the parts on both sides of it.
The piece at the bottom (screw end) of the band clamp is not much of an issue. Yes it has a fixed curvature, but it is pretty close per the pipe diameter, you are putting this on thin walled tubing so as you tighten it, the tubing conforms that slight bit to that curvature.
Last edited by Dave9; 12/01/19 12:39 PM.