I'm running the same set up. A SE 58mm t/body and a PM Gasser air cleaner...same as Rolands, or should I say Roland's is the same as daddys. My backing plate was the same as yours (designed for the factory 50mm t/body). Here is how I fixed mine for the 58mm t/body.
I bolted it on and broke a pencil into a 1" long piece so I could go inside the T/body throat and mark the inside diameter of the t/body onto the backing plate. I then removed the extra material from the backing plate. Ended up with a straight shot flow with the same size opening in the backing plate as the t/body mouth. I did mine in a vise (cardboard on each side of the backing plate so it doesnt get marred up)...I took a rounded rough rasp file to remove most of the excess material and then finished it smooth with a fine sand roll in a drill. It took me about an hour to do it. Turned out great.
117" Darkhorse build...all smiles!!!