Of all the suggestions given, I like Chris's solution the best. Moving your palm forward an inch or two will make a big difference with the 'artillery' type hold. Seach "how to shoot a B-28" in the China Gate. Mike gave a great post on holding the rifle at it's center balance point. I've used both, and have found my hold of the rifle is some what of a mix of the two holds. RF's idea of adding weight to the stock has it's merits as well. Definatelly worth looking into, and a hideable fix too!!!!
Personally, I don't think the B-28/Diana 350s's are really heavy,....just loooong. I'm not a large sized fellow either. If it were up to me, and with my particular shooting style, I would prefer the rubber butt-pad to be longer in the bottum providing more of a hook to catch under my front shoulder and bicept muscles. JMO, tjk