Brian,

Take a look at [link=http://www.rcuniverse.com/forum/m_2032835/anchors_2032835/mpage_1/key_yard%252Cbee/anchor/tm.htm#2032835]THIS THREAD[/link]. He's using a 400C system in the last picture, but the mounting should be very similar. FWIW, I probably wouldn't sweat 1/4" difference in thrustline location as long as you can preserve whatever down/right thrust is built in, and you have enough prop clearance. My word is hardly gospel, though. And again, if the whole system only weighs 5 grams more than the 280 system, don't even worry about it--the increased power will more than make up for it. On a plane this size, 5 grams difference in the total weight isn't even noticable in my experience.