I used the pillow blocks(not the kit) (got mine at northern tool) on my ultra heavy buggy cart, worked fine, try and get them as close the wheel hubs as possible, and I don't know if I'd put a middle bearing in on an off road cart without suspension.. I have 4 on mine now with suspension in the back, but without I only used 2, it worked fine, but more importantly the axles at northern tool have a certain amount of flex, it you put a bearing in the middle, it can't, and if you get airborne, you could end up with a bent axle (I did, but my brother in law got it 2 feet off the ground, and my cart weighs like 700lbs with a rider) so it was doomed no matter what axle I had on there..
I am also looking at making a solid axle by welding on steel trailer hubs and tool grade 1 1/2 rods. I am using a 1" tool grade steel rod for a jock shack, it is ridiculously heavier than the 1" axle that is 6" shorter! it has the downside of costing 35 dollars for 30 inches, and has no keyways, but its tough tough stuff.
beware, they are dry on delivery.. so make sure you grease them, too long running with no lube and you'll be tossing them out.