1980 Ford 555 Backhoe – Frame split question

After resurrection of our early 1980 Ford 555 backhoe about 4 years ago to use at my mining operation it has worked out superbly with reliable and economic performance, with many thanks to the valuable and generous help offered by members of this forum... Thanks Guys! Yesterday I saw something that I hadn't noticed before when climbing and descending some hills; the front section housing the front axle, hydraulic tank, radiator, counter weights, etc. was moving forward and back maybe 1/4" or more. I originally had to split the machine when first overhauling the shuttle and associated parts, so when reassembling I replaced the four very sad looking attachment bolts that connect the rear assembly to the front which had split lock washers behind them that didn't look like they were doing anything with square construction washers. What I found yesterday was that when tightening down the four attachment bolts all I was doing was cinching the square bolster guides that allow the frame tangs forward and back adjustment of the two halves. The square bolsters are wider than the frame tang slots. The frame tangs were not in contact with the front casting with maybe 1/8" or more clearance and not clamping to the front assembly. My thinking is that there is another type of outer washer that allows full cinching when tightening the four 7/8" bolts. I'm considering installing some flat washers with an internal diameter larger than the square bolsters that will take up the clearance, but any help or suggestions are greatly appreciated. Here are some photos that illustrate the problem. Note the clearance between the frame tangs and the casting when the bolts are tightened to about 250 foot pounds: Any help or comments appreciated. Thanks, -Tom F.