Why is the trailer too low?<br />Too low at the hitch?<br />Too low that the motor hits?<br /><br />Step down hitch inserts are available.<br />May have to tilt motor up for trailering.<br /><br />Raising the trailer (and boat) by 3 inches may make launching very difficult ( depending on the launches you use).
If the spring positoning lug is the same size as the ones on leaf springs on trucks, then any lifting block should work with the right length u-bolts, but I think that springs with a taller arch would be better.