here is what i did for my bh92. 8ft 4x4 and 2x6 all pressure treated. i used second pair of 2x6 under the operator seat where the stabilizer arms sit to distribute weight better. i used 4 inch wheels as many as needed to take the weight. your bh77 is going ti be lighter. but the critical area is under the stabilizers at the lowest point. that measurement will determine your max wheel size. to get length just make the arm vertical like your going to take the bh off and measure.
bh92 happens to hit about 8ft.
just make sure your back end where the seat is, is fully tilted up on storage or it will collapse over time meaning you will have to hook up the hydraulics at a wierd angle to readjust so you can move it around.
if you tuck it away and collapses the wrong way good freeking luck getting it out of a small tight area.
you want it like this not how i have it in the first pic. you want the back end as high as it will go so it wont bleed down over time. that sucked to fix:
