Simon, Merchant's mounts use two horizontal trapped poly bushings where the rubber goes on the OEM mounts. They are also made of thicker steel than the OEM. He's ran them on his shop truck for a while, and also beta'd them on other trucks, and he said the only time you notice them is on shut-down, where you get a bit more shake. Otherwise, no discernable diff from OEM mounts. If you go to his site, he has pics.
Sorry about the shameless plug, but both his and Rick's parts are really nice pieces. Heck, I only asked him to just drop an old drivers side mount in the box with the seals and such....and I got these instead. Boy was I surprised.
As for the downpipe, it clears the uppipes no problem now. When I was fabbing it up last year, I had to "guess" where the outlet should go, as I didnt have a mocked-up engine to fit it to. I really cant complain that its only an inch or so off. It wont take long to fix. If that fails, theres always Rick's LB7 downpipe....
We have a graduation party tomorrow afternoon and I promised my daughter we'd go see ToyStory3 this weekend. I also have a little prep work to do underhood (cleaning and paint) and I also have to go make my pre-lube setup. The motor wont get installed this weekend unless I pull a Trent-like all nighter which at my age just doesnt happen any more. LOL