[dsraven]

like said, don't bother shaving the mount adapter until you have a final height and placement resolved and an idea of what the new set up will look like. that adapter, if you look at it from the outside looking in, is simply a piece of round tubing with an L shaped flange welded on the end and is simply purposed to be an end point for the cross member tube. it is very easy to fab all by itself if you know what you want it to look like, it just needs to have the round tube on a flange so it will fit up against the frame and also accept the cross member. in the end, you want to be able to unbolt the mount from the frame and be able to remove the cross member for ease of engine/trans removal. this can look easy when you fab things up on paper or in your head but then the reality sets in. stuff like frame springing together when the mount is removed so you can't get the cross member back in without some sort of method to push the rails apart, or you bolt it together and then find out it is impossible to remove because the round tube is encapsulated inside the mounts and the frame would have to be spread the dimension of the round tube height in order to get the cross member clear of the round tube. I like the idea of the mount sitting on top of a flat surface because it makes it so much easier to disassemble later or shim up/down to get your angles fine tuned after the fact. you could use the web of the frame rail as a mounting point for strength, with a flat plate bolted to that. the flat plate could have a horizontal flat surface welded on at the correct height so the flat flange of the existing part in the pic could easily be bolted up. that way the cross member becomes easily removed if required but is bolted to the web of the frame rail for strength and there would be no cross bracing required to connect the upper and lower frame flanges so more room for exhaust pipes etc. as long as you don't have a frame mounted master cylinder set up that this idea may interfere with. if you estimate the frame bracket flat surface to be a little bit lower than actually required then you will have some room for fine tuning with shims to go up or take away shims to go down. your cross member is also bendable to get it in the correct place to fit your end point on the frame. a muffler shop with a hydraulic bender could do that in a minute for cheap as long as you can give them a pattern or a dimension.
get the driveline parts blocked where you need them (you can even block the engine/trans to be centered in the frame with a few 2x4's well placed and wired or cable tied) and then do some mocking up, with a normal trans mount installed between the trans and cross member, then the engine and trans can "flex" together as required under load/coast so that one mount system isn't trying to do all that work by itself (which is what could happen in your "rubber placed between trans and cross member" scenario). fab the connecting parts from crafter's poster board or cardboard and when you get it right cut them out of steel for the final.
on the engine height, I used a Camaro pan and it is sitting just about level with the bottom of my TCI MII cross member if memory serves me right. I can dig up a pic of that when I get home if I remember. I have my engine back into the firewall and raised slightly but it does sit pretty low compared to some I have seen. my trans mount is simply a piece of square tubing that is bent slightly to allow the trans to sit lower and it has flat bar welded on the ends, horizontally. then there are some flat bar pieces welded to my frame that those cross member flanges sit on top of. the flat bar on the frame is longer front to rear, than required to allow for any future engine trans changes. it would be easier to notch the firewall and do some quick sheet metal work for a trans hump that have to live with the driveline placed possibly not in the ideal location. the issue becomes what you want to use on the floor inside the cab because that would mean a custom floor covering instead of an "off the shelf" one. if cutting into your firewall make sure to cross brace the cab so it doesn't flex when the cut is made. otherwise you may have door fitment issues or cab mounting issues later. best to leave the doors on when doing this because then you will know right away if there is an issue surfacing. roll the windows down or cover them if doing any cutting/grinding so as not to cause damage to the glass.
just some thoughts for you to ponder while drinking eggnog...and checking ads in the latest classic trucks mag...and grunting appropriately to the wife's questions....haha