The 2 X 4 pieces held the plywood rigid.
Now, the longerons were set in place. I drilled the 5/16 in. holes where the ash met the spider plate and put some temporary AN-5 bolts in place. The cross braces held everything in place.
Also derrived from the Curtiss drawings - at 18 21/32nds from the firewall, the longeron should curve above the table 1 15/16 in. The triangular block was cut to that height.
Then, pull the clamps tight. It worked pretty well! I was pleased.
Next step is to taper the end of the longeron to 1 1/2in and cut for the splice. Till then...