Hi guys I still have my box section yamamoto from many years ago and I have the original box and plan. I live in Australia now so if it's worth your while I can help. here are the specifations that are on the plan.
Span 1422mm (56"
Length 1040mm (41"
Weight all up 2.1-2.5kg (4.5-5.5lb)
Engine 5.0-6.5cc (.29-.40 cu in)
Three channel or four if you add the aileron wing