It's been so educational I can't stand it ;-) Aha, I thought it might be a 1 5/16" nut. I ground my 32 mm crowfoot wrench out to 32.7 mm and it came off just fine.
You like a wrench like this?
http://www.summitracing.com/parts/sxt-942/overview/ You go for ? foot pounds? Using the 12 point side or the open side? As I happen to have a 32.7 mm crowfoot wrench and torque wrench in my possession, I am tempted to use them...
You are correct that the thick splines are clocked differently on the FD400 and FD500 arms. The FD500 was installed with the thick splines exactly 45° off of where they should be, so the FD400 will line up perfect.
To E350, I think we've seen how responsible this seller is. He's also 300 miles away and not returning my calls, go figure.