Lift blocks aren't the best idea on a trailer. I'm a victim of spacer blocks that left me sitting in the middle of a US highway intersection in a small town with one end of the front 6k axle broke the long U bolts and swung around to hang up under the rear axle.
As was mentioned a sub frame is preferred. I would also add cross bracing to the sub frame from side to side in the spring hanger area. This is a popular mod especially on the TT and 5th wheel forum. Their have been pics of the sub frame mods others have done.
My equipment trailer mfg says they will not use nor recommend suspension blocks nor are they a option.
Some years ago I approached a big rig trailer repair shop about adding blocks to gain more wheel well clearance height for 16" tires on a RV trailer. They refused ...however they said if I signed a liability disclaimer they would do it.
If you decide to use suspension blocks I would upgrade to grade eight 9/16" U bolts/a much heavier top plate and heavy spring keepers. Spring hangers would need to be be gusseted. And the trailers main frame rails at the spring hanger area may need fish plated or boxed depending on structural type.
Don't forget the tires on our close spaced axles in the center of the trailer slide sideways when going around corners. Adding blocks just adds more leverage (side stress) to the U bolts/spring keepers and top plates on up through the spring shackles and main frame rails.