8" trucks work great on an 8.25", however, I don't see the advantage of 147s and pushing out the wheels with washers (other than weight)..just ride 148 cast if you don't want magic carpet? ¯\_(ツ)_/¯
The lower top and bottom, clearly pull in the wheelbase/effect the geometry.
FWIW I've been running bones mediums (provided washer under the bottom bushing) in 147s with no issue at all, probably the best feeling thunders I've ridden in a long time (for the record, ages ago I ran 147 TI, with bones softs on an 8.25" Real SE before they decks hit market)...the shorter bottom does tilt the hanger in, shortening the 'thunder wheelbase'.