Hey Bruce... I can tell you what I have done on two frames so far with reasonable success. On a 10" wide foundation wall I bolt down 2x8 pt sill plates 3" in from the foundations outside edge. So if your frame is 24x40 your foundation would measure 24'6"x40'6". My 8x8 posts sit in a shallow mortise flush to the 2x8's outside edge and held firm with a strong tie strap. This of course means that the post overhangs on the inside by 1" but almost 90% of the post bearing area is still working and even more so on the corners. The reason for this 1" sacrifice is of course to provide a 3" shelf for the exterior walls. I have never used SIPS but I can tell you that with 1" T&G interior walls, 4" studs filled with extruded foam, 1" strapping covered with board & batten siding I'm left with an asthetically pleasing 4" overhang. A thicker wall would require a different approach. In my opinion forming in pilasters at post locations would be cheaper than pouring a thicker foundation for the entire circumference only to accomodate the occational post. Still out Almonte way?