The sloppiness in the steering might be due to worn-out king pins and/or steering linkage parts.
However, I bet that 90% of 47-51 trucks would also benefit from tightening-up the steering mechanism. This procedure is described in the Shop Manual and it sounds more difficult than it really is. You need to clean all the crud off the outside of the steering box, and you need to spend about $2.00 on a cheap fish scale (Wal-Mart has them).
Yes, you can safely go 55, if your front-end/steering and brakes are good.
Others will surely soon tell you have you can swap out engine, and/or transmission, and/or rear-end. It depends on how original you want to stay.
Sorry, I have no helpful advice about steering-wheel to belly clearance.
