Speedo Gearing question

[Cornwallrc]

I've got a Speedo Gearing question, Looking for some help.. I've swapped the gears in the rear Diff to 3:7:3, running a 350CI with TH350 Tranny, running 295/50/15 tires which = a tire height of 26.61" tall, My Speed seems to be roughly 40MPH off??? I think the truck originally had 3:4:2 rear end.. When I'm doing approx 55Mph its reading 100Mph Here's a picture I took along time ago "When building the truck" of the speedo gears.. Can someone help me out with possibly the propper gear combo I need to get or what my options are?? Thanks