That's a large change. The factory hole in the case on the OS .40FP is about 14mm (.55") "above" the engine mounting pads. If you kept your engine thrust line in the same place, you would be moving the nva discharge orifice by essentially that amount.
If you are pretty close, you might find the difference further reduced by using muffler pressure, and using a uniflow tank. You did not say what your baseline was, so....
If you want to rotate the engine, look carefully at your mount. At least on the Dave Brown mounts I have used, the hole pattern is not square, therefore your bolts won't line up except in the orientation it was initially drilled for. Not so simple as unbolt and rotate for testing.
You also did not say how much you did move the tank, and whether just a bit more might get you there? If this is an Imitation front end, then I can see the problem of the tank thru the fuse.
One thing to consider--if you are not allergic to plastic tanks, as some people seem to be--is a Sullivan clunk setup where you can move the uniflow point up or down and not have to have the tank body centered perfectly.
With a firewall mount (plastic R/C mount) you must be aware that you are trading one problem for another. You will also be moving the orifice laterally by 14mm. The engine may tolerate this ok, or it may not. I worked on one OPP (Other Person's Plane, gifted to me) that used an R/C mount and sideways engine. With the fixed, internal tank (no access) you couldn't set it lean enough on the ground to get a flight setting that wasn't stupid rich. Yup. Had to cut into it and get the control line engine/tank relationship back. On this one, I chose to keep the tank inside the fuse of a heavily modified Fancy Pantsy, but rotate the engine to vertical. The little OS .25FP just couldn't handle that much lateral offset on what for it, seemed to be a fairly wide tank.
If you simply cannot bring yourself to move the tank further, then you might try what I had to do once on another OPP: I took the R/C mount off, and made a donut shaped "butch plate" out of 1/4" aircraft plywood. It had two sets of holes. One set lined up with the blind nuts buried in the firewall. Flathead machine screws (countersunk) went there to hold on the plate. Then I drilled another set of holes to attach the mount while relocating the engine thrustline--and therefore the nva--to establish the correct tank height. I intentionally offset it a bit so that some shims under the engine lugs were required. I made up a number of these so that I could tune the setup without endlessly making and remaking the butchplate. Worked out fine, considering what I started with: a plane that would fly great upright and on the insides and would instantly go rich and die inverted or on any negative-G maneuvers....
Good luck with whatever solution you find,
Dave