Thrust line is always critical. Get it right, as per the manf. recs. You can always knock the old blind nuts out by putting long bolt in them slightly, then tap then back into the fuse. Seal the holes with tooth picks, or small dowels and epoxy. Then install the new, different mounts per the mount manufacturers instructions and keeping the thrust line at the plane builders recomention between 12 and 6 o'clock and 3 and 9 o'clock. This usually means some right and down thrust, so the mount may be off to the left and a little up on the firewall, so the prop center is on the proper thrust line. If the model is statically balanced for and aft and laterally, then fly a vertical up line and check if it goes right with throttle increase. at level flight, if it rises drastically on throttel increase, you need more down thrust. If it tries to turn left on throttle increase, then you need more right thrust. I recently bought a new U Can Do .60, that had the firewall improperly installed with a left thrusted engine. Thought GP knew what they were doing, and had a real surprise in the air. I am now one degree to the right and about 1 degree down and feeling much better now. Hope this helps.