Roger,
No, I did not use the soft isolation mount with my DLE 55RA on my TF P-51. I used the standoffs that came with the DLE 55RA. I used 1/4" aircraft ply to get the correct distance for the engine to line up with the nose ring of the fuselage. It takes four 1/4" layers of ply. I also reinforced the bulkhead to the fuselage sides, top, and bottom on the inside with 6 oz fiberglass cloth. Here's a picture. (The mount is not angled, it just looks like in the picture.)