My UCD 46 has a OS 91 2C and I had to add 4 oz of lead to the tail to get it to 6 inches. I would try to install the engin last and try to get the CG to the aft piont. then use (less than me) lead to fine tune it. You will not have much to work with because the cowl will only go on so far. Try to give your self some adjustment with the CG. I did not like my UCD untill I got the CG back. Now it Rocks. I an not sure i would have had a good maden that far back. The time to fix the ailerons will pay off.