Best thing I can recommend if finding a place that has the 295/40R20 in stock. Many places will do a free test fit if they have the tire in stock.

295/40R20 (29.3") doesn't sound tooo extreme. I'd say the chances are good that even at that new offset they'd fit. Probably need to trim the corners of the front fenders in front of the tires a little.

Only a test-fit can say for sure. Every truck sits different, so sometimes something that works for one won't work for another also.

Good luck,
-Daver