88 F-150 Cab Loose from frame

I have an 88 Ford F-150 pickup.  The cab is loose from the frame on the
drivers side.  The reason is because the floor rusted out on that side,
and there is a reinforcing bar, which is part of the floor.  That bar
also rusted out and is the part that is connected to the frame.
Apparently from moving around, the rear rubber mount on that side is
also halfway slipped out of the cab.  

I bought the truck this way.  I knew the floor had been patched but
never noticed the cab being loose.  Besides this, the truck ran well. I
have to replace the rear brakes, a u-joint, and a couple door latches.
Otherwise it ran perfect.  The reason I'm posting this is because this
is more than just an annoyance, which causes the cab to shift when I hit
bumps in the road, and occasionally the drivers door wont latch without
lifting on the door.  The real problem is that the truck dont steer
right.  Taking it up to 55mph, I nearly went into a ditch right after I
bought it.  The steering was erratic and almost uncontrollable.  I took
the truck to a shop to get the front end aligned.  The guy asked me what
is wrong, and I told him about the erratic steering.  He checked the
alignment, steering parts, and front tires and said everything is fine.
That's when I mentioned that the cab is loose, and he told me that is
the cause of the problem. The steering column moves with the cab and is
not working with the frame at the same time.  He told to take it to a
frame and body shop.

Actually, this is only a farm truck which was sold for a low price.  I'm
sure a body shop would charge more than I paid for the truck.  I'm
handy, so I figure I can fix this myself for not a lot of money.  My
question is whether any of you have run across this same thing, and what
you did to fix it?

One guy told me to just bolt a piece of treated 2x4 to the frame and
fasten the floor to it.  I suppose that would be better than what I have
now, but I'd rather do something a little more secure and made from
metal.  Any ideas?

Thanks