Hey Monkey Boy,
I measure my restored Holeclaw trailer and found the frame to measure 51 1/2" above the axle.The distance between the spindles is about 56/1/2".The wind is really kicking up right now so it was hard to give you a better measurement.
On the front& rear cross bar,I installed a 96" long 2"X2"X3/16" steel tubing.
Once you finished drilling holes and whatever to your crossbars,remove them and paint inside that tubing to really deter rust for starting inside out.Use a 10' long piece of 1/2" PVC pipe with a rag wired and screwed to an end as a paint brush.Like a long Q tip.Do this to all the tubing .I have restored many trailers and used several very expense paints before and found that the best paint is Rust-Oleum oil-based(Professional High Performance Protective Enamel).It will dry quick and last and cheap too.
For the front rollers,I went 79 1/2" center to center.
For the rear ,I went 81 1/4"
Those roller are called 8" but measure a little less.
The original rollers were less than 6" and did not fit the boat good,so I cut the bracket in half and added a 2" piece of steel and welded it together.The 8" rollers work well.
I like my cross bars little longer so I can use the extra space as a step to climb on my boat without stepping on the light fixture.
I run all my wiring in vinyl tubing and through a rubber grommet as it past into the steel tubing.No one wants to trouble shot damaged wiring while on a holiday trip or get pulled over after enjoying a few cold beers at the lake......
Good Luck,Bill