Bob, I have a '50 3100 also and it is a stock frame except where it's rusted bad enough to need plating. While it seems stiff it does flex some. I would at the minimum box the front end up to the cowl but make sure your spring shackles are not in need of repair first since you will be covering their mounting rivets on the inside.If it's feasible, just box the whole frame and be done with it. The 292 will have way more torque then the original 216 so you need to reduce as much flex as possible.
