Holly Cow - A Saito 150!
PWRDrill - I balanced mine at 1/4" behind the spar. I put my batt. pack next to the throttle servo. You may need to put your batt. pack in the tail section. Mount your Saito 150 as close to the firewall as possible. I would install oversized tri-stock behind the firewall. I would also drill the firewall about every inch along the sides in a 45-degree angle to the firewall, through the firewall, through the tri-stock and out the fuse side. Install 1/8" dowels using 30-minute epoxy. Once dry, sand the dowels flush with the firewall and the fuse sides. You may want to consider installing 1/16" balsa sheet between the first and second wing ribs from the root of the wing top and bottom. I would think that this is optional.

Here is a quote from another Ecstasy customer of mine. I hope that he doesn't mind.

"Today the xtc took to the sky. It performed beautifully for initial flights.Did waterfalls , flat spins both upright and inverted, and all the standard stuff. It is a little nose heavy at 1/4 in behind the spar. I have changed that by moving the battery to under the wing tube. I may need more rudder throw also, only about 30 degrees at the present. It hovers very very easily. Will have you pictures tomorrow , in fact will have some E-mailed to you. I forgot my camera and no one else had one. The nose still looks funny to me,however, everyone else likes it. Will let you know more tomorrow. Incidentally, the weather was gorgeous, low 70's. wind 5-8 mph straight down the runway. Florida does have its advantages--sometimes! Pratt "

As least Pratt offers you another opinion.
Hope that this helps.
Let me know if you have any other questions, or I can be of any further assistance. I am curious to hear how your Ecstasy comes out with the Saito 150 (I still can't believe it!)
Kevin