I asked Jim about his mods earlier this year. This was his reply:
First of all, don't use the plastic top piece and scoop.... Mold the top from 3/32" medium balsa and on the bottom use a piece of 3/16 or 1/4" front to back with a scoop made from a box of very light balsa, carved and sanded etc....
Core the wing in a standard three core fashion with a suspended bellcrank. The platform the plan shows is heavy and prone to failure....
We did not use full span flaps, making the outer 2-3 inches fixed.. Or adjustable on the outboard piece...
And then... Todd's mustang has a 15" tail moment, hinge to hinge and mine is 15 1/2" - which may be a bit too much... plus the stab/ elevator are enlarged. 22" span with a stab at 3" wide in the center and 2 1/2 at the tip. the elev is 2 3/4" at the center and 2" wide at the tip...
Remember that this is a rather small airplane at 490 sq in.... We both used Fox 35 ... And in the 38 - 40 oz range this is good power.... Gotta be very careful with the wood selection and use of glue....
Equal span flaps, and try to get the asymmetry to about 1/2 - 3/4".. lengthen the outboard ( bigger tip block?) rather than shortening the inboard...
In the mid seventies Arlie Preszler did a piece in Stunt News about lightening the Sig Mustang, which is where we started... they were using it in WAM competition on the west coast with Veco 19's...
email again if you have more questions...
Hope This helps...
Jim