1. There have been so many damn models of the Sirrus over the years

2. What one maker calls a 32 can often be a 34 or 30. Plus this measurement won't include protruding teeth. So this gets messy.

The OP's best bet is probably to try fitting 35mm Duremes. (If he is willing to pay the price. Otherwise for "hardpacked dust" he could try cheap but sticky 35mm slicks.) Hopefully they'll fit and they're a tolerable width for light off roading as long as the rider isn't too heavy. Fortunately hardpacked dust is an easy ride as long as there isn't bad rutting.

Even for a heavy rider 35mm is ok as long you're willing to put up with being banged about and steer around or directly over every tree root, hard rut, and small branch. I'm 220lbs and my crosser has 35mm tyres on, I spend a *lot* of time off the seat using my arms as suspension. I can ride light MTB trails and do it fast, but riding them on 35mm requires total concentration - taking a small root or rut at the wrong angle can mean a wipeout.

The difference between 35mm and 40mm doesn't sound much but the volume of air in a tyre is proportional to the square of these sizes - so a 40mm has about 50% more. That gives you MUCH more suspension. Oh - and the reduced traction of a 35mm makes cornering and braking much "interesting" too.

Measure the bike for tyre clearance with a ruler - the chainstays are usually the problem. And then order. If the tyres you order won't fit, then just send them back - a decent store should be cooperative. And measure the side of your rims and check they'll take 35mm tyres with the chart here:

http://www.sheldonbrown.com/tire_sizing.html

- There shouldn't be a problem with this, but it's smart to check.