Soft Sand & Tires

I have a 1997 Cannondale hybrid I use for beach crusing ONLY and I want
to make it the most efficient for getting through soft wet and semi wet
sand which is like ridding through glue (good workout for the knees
though). I am constantly is my lowest gear and when I get a little wind
in my face and the sand is real soft it stops me in my tracks. I've
ordered a new cassette with larger sprocket but I want to get new
tires(I have 700c's, 38mm front and a 42mm rear now( not knobbies but
some tread). I'd like to get a pair of 47mm wide slicks. The problem
is the rear tire will fit between the chain stays(frame) but with only
a mm or 2 of space to spare. If my wheel were to go a little untrue or
the tire gets slightly deformed it might rub against the chain stays.
Is there a minimum amount of space between the tire and the stays that
you go by? COULD I FILE DOWN THE CHAIN STAYS A LITTLE? Are the chain
stays hollow or are they solid aluminum? Do you think these 47mm's
are the best choice or should I just stay with 42's? Should I get a
little tread for the front tire? And finally, what pressure should I
fill them to to get the lowest rolling resistance for wet sand?. I
weigh 200lbs?
I have read Sheldon's site on tires and I'm not sure what he means
by "Narrow" back. How much narrower? This is from Sheldon's site.
Hope he doesn't mind me posting it here.

· Off-Road Issues
Bikes that are used some of the time on loose surfaces often benefit
from a wider front tire, with a fairly agressive tread, coupled with a
somewhat narrower, smoother rear tire.
The wide, knobby front tire will provide the all-important front wheel
traction. If your front tire skids, it almost always leads to a crash.
For riding in soft conditions, such as sand or mud, a wide front tire
is essential. If the front tire sinks in and gets bogged down, you're
stuck. If the front tire rolls through a soft patch OK, you can
generally power the rear through to follow it.
The narrower, smoother rear tire will have lower rolling resistance.
Since most of the weight is carried by the rear tire, rolling
resistance is more important on the rear than the front. If the rear
tire slips, in most cases the worst that will happen is that you'll
have to get off and walk
http://www.sheldonbrown.com/tires.html

One last thing, here's the tires I'm looking at. Harris doesn't
have 700 x 47 on their site. They are hard to find.
http://www.bicycletires.com/tek9.asp...cific=joopdpg8

Thanks