OldIHC

I've been thinking about widening my bed on my D2 so it can fit a 4' sheet of whatever? Anyone done this?

I just had a look at a D2 box. There is no easy way, although it is doable. The width of the box will not match the cab after this misadventure. The original value of this truck could be reduced many times.
If you are not hauling 48X96 plywood often, it might make better economic sense to have aa few sheets delivered.
The box I looked at has a steel floor. I think it might be easier to modify a wood deck box. Making it look right will be the challenge. The biggest challenges will be modifying the tailgate then to a lesser degree, the bulkhead.
If this were my truck, I would not make this modification and I have the tools and the trade.