Vixen exhaust woes

[GJL]

I have a 1971 Vixen 2500 and I am fed up with trying to stop it blowing at the manifold to downpipe flange. I have tried different types of gasket, new studs, brass nuts, extra long brass nuts, steel nuts, spring washers, nordlock washers etc. I also welded a braided flexi pipe in after the bend in the downpipe to eliminate some vibration.
I think the problem is that the middle and back boxes take a smack off the tarmac on the poor and undulating road surfaces around here.
I could chop the 4 inch silencers out of the system and replace them with 3 inch round silencers. Or delete one silencer and have a 3 inch diameter silencer at the back in line with the wheels, so hopefully it will be less likely to contact the road than the middle silencer.
Something like this... https://jetex.co.uk/product/round-silencer-box-mic...

I am running on Toyo CF Proxes 185/65/15 tyres. They are good tyres but I could change them for something more akin to the original size as perhaps the lower 65 profile is causing the car to bottom out.

I have set the ride height and stiffness to what is an acceptable level and didn't really want to go any stiffer. I suppose a tyre with a taller profile would have more give and soften the ride slightly so I could adjust the shocks a little too.

Any thoughts or ideas will be appreciated. Thanks in advance.