Panaracer has the "Fire Cross" tire that's a really big 'cross/narrow 29'er tire - it's a 45c. YOu may have a tigh squeeze with these, depending on your frame.
If you can find some Rithey Zed's, those came in a 42c, too. I've run the Zeds, which are great but rolled the bead off of my narrow rim today on a mini-motocross track. Great tire, I need wider rims.
Also, the Ritchey Speedmax's come in a 40c width. I've run these on some MTB trails with success, but you can't run them as low as you might for a 'cross race.
Chris' suggestion forthe top-mount brake levers is a good one, too.
I like a bigger cassette for those long steep uphillls, butthis will depend on your trail and your strength/abilities.
awesome info. i tried the fire cross and it fit up front but not in the back. i run a felt f1x which has pretty good clearance. the seatstays were fine. the huge side knobs didn't clear the chainstays.
i have just ordered up some continental cyclocross tires which come in 42mm and have a less aggressive tread so i'm hoping they will work. continental also makes a file tread in 42mm which i may give a try later on when the trails get super dry and like velcro :)
i do this all the time - been racing the local series on my cx bike. a few more tips:
1) not sure what drivetrain you have but an mtb cassette helps for the steeps/long climbs. a gs mtb rear derailleur works well with this. there's an alternative cable clamp to make 10s shimano levers work with 9s cassettes if you're running 10s sti.
2) top mount levers are good only if you have them mounted wide, IMHO. get the ones that mount on the narrow part of the bar and put them wide. you have no control if they're clamped on to the bulge. getting a wider bar even helps.
3) other tires to look for: mount cross (not made anymore but my fave for trails), wtb had a mutano 44c tire not made anymore but can be found, the ird mythos 42 is good, and there's also the wtb interwolf (i see this for $10 on nashbar sometimes).
4) grifo's run true to width, unlike some other brands, and are nice and supple but if you're out with rocks, the sidewalls are a bit more fragile. the hutchinson and duro tires are even larger in volume than the grifo - duros are about a 37c.
5) not sure if chris got the link wrong but bonty makes narrower 29er tires than that one. they have 1.75, 1.8 and 1.9" tires. here's the 1.75" one: http://bontrager.com/model/07573. we're currently testing some of these.
6)we have some content coming in this area too in the print mag. bottom line, have fun on it. at least out here, it's a great way to go for most mtb races since there's so much fire road. it keeps it challenging, and the light weight really is great on the climbs!
I run the Fire cross on my Redline for MTB trails. Trim the side knobs with dikes and you'll gain some clearance. I also use Tufo 32's. Pump them up to about 60psi. I've got a posting here somewhere called Monster Cross that has pics.
You may have already dealt with this, but the major piece of advice I'd give is make sure you're comfortable in the drops. I find myself in the drops a ton on technical trails, partially to get a better grip, and partially to give myself more leverage on the brakes.
I also run my brakes a little farther from the rim, so I can get my hands on them more easily. If you have big hands or reach-adjustable levers, though, that may be less of a concern.
i got in my continental cyclocross 42mm tires and they fit front and rear. i took them for a spin on our local singletrack and they worked great! the tread pattern is similar to the mountain king but the lugs aren't nearly as tall. good traction on the rooty, rocky, dry and loose dirt of north carolina. for posterities sake i thought i would throw out the measurements on my bike and the tires so you can see if they will fit your frame. i have a felt f1x and i measure 4.5 cm from chainstay to chainstay at the tightest tolerance for the tire and 5.1 cm on the stock fork. the 42mm continental cyclocross tire is 4cm at its widest point and didn't leave a lot of clearance but not too bad. picture below so you can see. the 45mm firecross measured 4.7 cm at the widest and did not fit the back but did comfortably fit my front fork. i would put the firecross up front but i have to loosen my brake pad to get the wheel on and off which is a bit of a hassle and the conti works really well so....