I replaced the K member in my Charger in May. I found that the RH motor mount was broken at that time so I scrounged around my stash and found a decent used one. The rubber portion looked thicker than the old broken one but I figured that the old one was just compressed and worn.
The thicker RH mount raised that side but also moved the LH header closer to the steering box. Because of that, the header would now sometimes make contact. Looking at the LH mount, it appears that it is thinner than the right by a noticeable amount.
I estimate the thickness of the LH (drivers side) mount to be 3/4" to 7/8" and the RH side to be 1" to 1 1/8". I would simply switch them side to side but the LH is one of the interlocking types to avoid failure. Looking at the Schumacher (spelling?) site, the pictures they show make their mounts look at least 1 1/4" thick. Today I checked at a local parts store and found that their replacement mounts are at least 1 1/8".
Have you found a variance in thickness of your motor mounts? I found that the 66-72 B, 70-74 E and many C body cars use this same mount.

http://www.engine-swaps.com/Pages/ProductsYear/66.72bBody.html#head2

I thought maybe I could move my LH mount to the right and install a new Poly-Lock one in the left side. The goal here is to get more clearance for the header. Before the K member swap and RH mount replacement, there was plenty of room there.

I have a set of the schumacher poly mounts on my 440 truck

they seem to be a lot thicker than 1 1/8" I will post a pic later and put a tape measure on them.

I got them cheap from member Dragula, he said he did not care for them for some reason? I installed them and have no issues with them. no vibs, no engine movement, ect...

I know some of the truck guys talk about the a thicker mount for the diesel trucks that works good on a gas engine truck. may check those out as well.

the metal/solid mounts I looked at at the speed shop did not seem to thick at all compaired to a rubber mount, maybe a 1/16" difference. MOROSO brand IIRC.

I have cut up square pieces of old bias-ply tire to make a stack/mount with thru bolts to make xfer mounts on my 79 RC. fugly but work.

They could very from one manufacturer to another. I know that on small blocks you have to watch out for car application and truck. Big difference. I've seen on E bag that some people will post truck, saying it's for car.

Yes, sometimes there can be a noticeable difference. When I first installed the engine in my 'Cuda I bought new "genuine made in India" mounts from my local NAPA store. A couple of years ago I found that the RIGHT side mount was broken ie. the rubber had separated from the steel. I was surprised that it was the right side since usually it's the left side that fails due to engine torque. I have a torque strap so maybe that's what saved it. Anyway when I went to replace it all the new ones were at least 1/8 in. thicker. This caused the right side header to contact the torsion bar since now the engine wasn't sitting even. I knew changing the left side would be a bit of a PITA so I was able to loosen the left side and slide a couple of metal shims in that side so everything is even again. FWIW the new mounts were also made in India.

Maybe I'll just make an 1/8" metal shim plate and slip it between the mount and the K member.
Free and easy....Like a few girls that I knew many moons ago.