This is fairly straightforward. The "dash" frame is mounted
as shown in the drawings and glue blocks are used for additional
support. The center strong-back is let into the stem with a tapered
joint as shown rather than a simple tapered lap which would have left it
To measure and cut the tapers:
Place strong-back in position over the frames. Set rear end flush
with the dash frame. Mark the rear edge of the stem top cap on the
frame. Trace around the forward end of the strong-back on the stem top
cap. Taper the strong-back from the mark to the end.
Press down on both ends of the strong-back and measure the gap
between it and the rear top edge of the stem top cap. In my case this
was ~1/4" Mark the rear edge of the stem top cap down from the top
for the thickness of the strong-back minus this measurement. Use a
handsaw to cut the sides of the tapered notch as well as a couple of
cuts in the middle and clean out with a chisel. Once the other
notches for the strong-back are cut, it should drop in place with the
tapers matching nearly perfectly.
The horizontal pipe clamps were pulling the rear most deck frame
straight. Do not assume that they are straight. This is your last chance
to fix it!