I dropped them both down from the top, copper rtv will likley help, my issue is the gasket kinda swooped down and just caught the edge of the flange and blew out right away, I ended up using a mr gasket ultra seal gasket there.. the other side was perfect though, and i'd wait on the clips and such for the locking bolts, the allen head on those will save you many knuckles and curses though. just put them on tight, run it good and hot then tighten when its still hot, let it cool completely and retighten what you can, run it for a week or 2 then repeat and you should be good, also you will likley have to use stock bolts with the spacers provided for the front 2, to clear the a/c (if you have) or the locking bolts may work as well. just be sure that they are not too wide with the washer and all, theres not a whole lot of room.
and that should sound really good.. I need to get something as dummy cats, i even though of 2 mufflers in a row, or a 22" magnaflow
are you welding in O2 bungs (not sure what year your truck is)