50's stepside on 60's truck

[Short 3/4]

So the good news is I found the short stepside bed I've been looking for for a year. An old friend just across town has one he'll sell me for a very reasonable price. It's in great shape and even the right color! But... It's from a 55-6-7 and currently made into a trailer. It appears that the frame rail width ( aprox 32" ) will work, but the front to rear measurements of the four cross members don't line up.
My question is, how many cross members should there be, and what is the correct spacing? Is this hard to change?
There was no bed on the truck when I bought it, so I have nothing to go on.
Truck is a 65 K10 that was originally a stepside, but that bed is long gone. Any help will be greatly appreciated.
Thanks, Tom