Those repro finned clamps are not strong. Be careful to not tighten the nuts too much.
When I'm refitting my downpipes I make sure the exhaust valves are shut then scrape out the ports and blow the dust out with the airline. I clean the pipes with wire wool if necessary.
To seal the pipes when refitting, I use motor exhaust sealant paste, spread thinly on the pipe and in the port. I use a rubber hammer on the main curve to tap the pipes fully home. I found I had to pack the brackets that attached the downpipes to the engine mounting through bolt. The silencers fit just fine, although the downpipes I bought, from Fecked are long and the silencer deep-throats about five inches of pipe.