The .35 Whelen should lose less velocity per inch than the .30-06 does. In a test from the November 1981 issue of G&A the author found this as he cut back the barrel of his .30-06 rifle firing 165-grain bullet:
26" 2896 fps
24" 2850 fps
22" 2820 fps
20" 2772 fps
The average is a loss of 20 fps/inch. Cutting back your 22" barrel to 18" would lose about 80-100 fps tops. This is insignificant for any practical use...if the shorter barrel enhances your hunting, it more than offsets the tiny velocity loss from the shorter tube.