Tire size

[MikeIIDC@domain.elided]

Okay, 

Went out this afternoon to get new tires for the Wagonmaster.  I have
limited funds right now, so I was looking for something reasonable,
which is unusual for me.  I found something, finally, but, of course,
the local yokels at the parts store didn't know a whole lot.  Anyway, I
have 32x75R10.5's on there now.  Wheel is a little wide-10" I think?  I
guess, after talking to them, that I had the measurement system on tires
all wrong.  Can anyone tell me where (on the internet, hopefully) I can
find a conversion chart for the P235 to what in inches is?  They have a
book on it, but couldn't find it!  

Anyway, these are for the '74 100 Series Wagonmaster, so maybe some of
you with stock Travelalls from this era could help me.  This is a 2WD,
so I want something for the highway, but in testing tire sizes, I put my
31's on there and they look pretty cool.  What is the largest tire that
you guys have been able to fit safely on your T'alls?  The old tires
were 32's and were rubbing, so I have to go down to 31", but these are
about the perfect size, so what radial (or passenger car) rating/size do
I look for.  The only thing they had in stock that looked about the
right size was a 255x7R15.  Holding them up next to the mounted
31X10.5R15's that are mounted, these looked about the right size.  How
much height will I lose having them mounted?  I want the thing to have
the wheel-wells full, but not ridiculously so.  Do these sound like the
right size?  

And, if anyone has the patience at this time, please explainthe sizing
system.  I thought the second number was for the thickness (width?) of
the rim, and the last was the diameter of the rim.  The first, I was
thinking was a circumferential measurement.  Could someone set me
straight on this?

Thanks for all your help/info/knowledge!

Michael