I have the sleeper kit. Not installed yet. But, yes it looks like you would have to cut the front part off the catback and just weld it there or clamp it.
Yes there is some sway if you leave full size radials on the front... but with skinnys its much better.. the only sway is when you let off hard at the end of the track. But your prepared for it. I pump them up to 23psi on the street and they are much better.
M&H has a few horror stories of blowouts... probably cause by the wrong rim size like you say..lol
We dont have many options with 18" feont runners. 2 options I know of. A 26 or 28" radial. I run the MT 28" radials on mine with bias ply rears
Aldo welds kit is nice and well worth the money for what you get. LPF also has a good kit. I actually went with the hellion sleeper kit. All 3 are good. imo I wouldn't consider anything else for twin kits.