[BikerPepe`]

If they aren't seating snugly against each other, I'd be considering that rust over the years has formed where they join together and it's preventing them from seating tightly against each other. You might have to take them apart and file them clean/smooth. Not sure what else you could do. Maybe use some aluminum gasket material between the two? Maybe from some those stack-able metal exhaust gaskets? I'm spit-ballin', of course. I know the ones on my old F250 had come loose from use/vibration and I had to re-tighten them. I used the steel lock-nuts the second time to keep 'em tight but they seated fine and being long tube headers on an old work rig, I was less concerned about minimal leakage.

 

[BikerPepe`]

Funny to see this pop up again. I ran into a similar issue on Dad's 400. Flange had rusted and rotted away enough that I ended up buying a flange replacement piece, custom cut/match and welding it in to replace. Worked out nicely. Not so pretty but I had to weld it in place.

 

[BikerPepe`]

Wish I could have found one of those before @jermil01 Luckily, my lil' lady bought me that welder last year.

Just for the record... sorry it's a little fuzzy but this is the flange replace piece I picked up at AutoZone:

With the bend in the exhaust pipe to meet the manifold flange, you can't just slip it on over the existing pipe as it was designed...

So measure twice, cut once and butt-weld it back in-place...

I'm not proud of my welds but it is aligned, solid and effective. DIY Tip... a flashlight ran around the pipe, exposed a small pin-hole I missed at the very bottom.