perch and block in one
You can do something like NEW INTERIORS did once you know the length. Another way is to get a 2x5 or 2x6 cut off the usual length of a lowering block 5 or 6 inches, then measure dead center of the piece, drill a pilot hole through both sides, get a 3 1/4 hole saw and cut your circle. When your finished cut it in half, now you have two identical ones. You can box the ends to make it stronger. Don't forget to drill the hole for the locator pin on the spring. In some cases the rear axle needs to be re-centered anyway, so you can control where it goes too.