LAHD Drawer Slides

I'm working (in stages) on a big work bench and storage unit to go the
full length of the back wall of my shop. I originally planned a single
monolithic steel top bench 56 feet long. I've since decided to make it
semi modular because somebody will want to disassemble and sell it.
Probably at the estate sale after I die.
I also asked my wife if she had any ideas about how to get more out of
it. She made what is actually a fantastic suggestion. Put 6 or 8 foot
long wing benches in every 8 feet. This allows me to my 8 foot modules
with drawers one at a time, and in the long run will increase the bench
top surface by nearly 1.5-1.8 times. Sure that's more surface are to
suffer from HSD (horizontal surface disease), but it means its also more
space to pile up things out of the way to make room for the current
project.
I have some steel tube (its pretty expensive right now), and I have 1/4
inch steel plate for the entire project.
Drawers are the problem. I can make plywood drawers (probably one or
two at a time unless the price comes down) that will easily manage
hundred of pounds. More specifically though drawer slides are the
problem. I recently found a source for 250lb 30 inch slides at a pretty
reasonable price. I think the seller wound up with a huge surplus lot.
I've purchased 32 sets so far, and will purchase a probably that many
more. Now what I am looking for are LONG ASS HEAVY DUTY drawer slides
for the wing benches. I think 60 inch is a minimum, and 72 would be
better. I've found a fair number of 60 inchers. The good news is every
set I've found is rated for 500lbs. The bad news is every set I've
found is relatively expensive. I may not do all the wings that way, but
I hate to create dead space in the middle of all this investment either.
I'd like to setup at least one 6foot wing with slide out drawers/trays
for storing six foot stock. Long stock will probably still have be
stored outside for now.
Now before any of you all go off and tell me about buying a hodge podge
of desks, file cabinets, and bins at auctions and yard sales... for the
purpose of this I just don't care. I want something better than that.
I have a hodge podge of stuff already, and that will probably all go
upstairs over my office.