Touring - heavy duty wheels

I am starting my first long distance tour in the first part of may. I intend to start in southwest IN and go north till I meet up with the northern teir route,and follow it west. since I have never rode this much before I am taking it easy ,only hoping for 50 miles a day. I got laid off from work for the second time in 2 monthes so I figured "what the heck,why not?" I have wanted to do this for years and have most everything I think I need .except last year I sold my 48 spoke wheels I had built for my mountain bike. actuallty I sold the whole bike! anyway, I have been building a new bike for this over the last month and I am a little worried about my budget for wheels. a new pair of 48's (shimano tandem hubs and ryhno lite rims)is between 400 and 500 bucks ,depending if I buy them online or have the LBS build them.I have been reading on here of the strength of well built 36 spoke wheels,and after comparing the costs it would seem a cheaper alternative.
I am 6'2" and I currently weigh in a little over 300# and I figure around 50 # for my gear and I am not sure what my bike weighes yet, but lets say another 50 #. could well built, 36 spoke wheels(lx or xt hubs and rynolite rims)hold up to a 3500 to 4000 mile tour while rolling a fat guy and all his stuff along?.LBS guy seems to think it "could"but I should be prepared to have it looked at every couple days or once a week ,or ? he thinks ,and I know ,with the 48's I wont have any worries. for the price differance i fell like I could have a least a month longer on the road with out worring about money,so its pretty important.
LBS guy suggested building a 40 or 48 for the rear and leaving my current 36 in the front ,providing the hub is in good enough shape. any opinions on this subject?thanks for any advice

It depends where you travel, how you ride and how much you carry. You currently have 200 lb of yourself plus 30 lb of bike plus 25 lb of gear (assuming you split evenly front/rear) on the rear wheel, which means a total of 255 lb.
Last Summer, I toured with a trek 520, with 80 lb (bike + gear) plus 100 lb (me) plus 10 lb (daughter) on the rear wheel, for a total of 190 lb, and had no problems with my wheels. In fact, the rear wheel has stayed true and in good shape for about 16 000 km, in spite of our numerous potholes, cracks... Then the rim tore apart and I had to replace it. Hub is LX 36-spokes and rim was Borntrager off-centre rim.

There also are sports tandem teams that ride with 36-spokes wheels.

So definitely, a well built wheel would work fine. Especially if you lift your butt on bad bumps, if you pedal smoothly and don't stand up and throw the bike on each side when climbing.

If you are looking for strength in a rim the Sun Mammoth IMHO is the way to go. LX or XT hubs certainly is a cost effective hub, however DT Swiss Onyx s a better hub and worth the extra dollars.

how about the quality of pre built wheels from nashbar? I got the new catalog to day in the mail and they have a set with deore hubs and sun rims for around $135. I hate to go around my LBS but for the price it might be worth it.

You are a big guy indeed.If I were you I would have taken the LBS suggestion i.e. leave the front 36H and get a new 48H rear wheel.
IMHO the wheels are the most important part of the bike and one shouldnt try to save money there.