Actually, I have not seen a trailer raised/lowered that way--I have only seen spacers used between the frame rails and the spring mounts, or the axles placed over or under the spring mounts to change the height. Suggest you find a steel supply shop and look at square tubing, or bar stock.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired