I'm not clear on what your problem is. I also have a '48 and had trouble with the width of the boards. I wound up shaving a half inch off one of them. It sounds like you're having a problem with the length of a board, though. Is that the case?
I suggest putting all the boards and strips in without tightening the nuts. That way you can move everything around as needed to make it all fit. To do that, however, you'll probably have to lower the tank.
I saw a uTube video of one guy who put his entire bed togather off the truck, then positioned it on and bolted it down.
