Just like they do as on smaller machines.
Most of the bigger machine tool builders in their times had templates that fit the profile of the bedways and were maybe two to three foot long. These templates were scraped using straight edges to get good bearing points. Once that was done, they used the template to scrape in the bearing points on the beds of the planer. The bed of the planer was dead straight when taken off the larger planer it was planed on. They heavy scrape the fresh planed finish just like Richard teaches his beginners in his class. Then they come in behind the heavy scraping with the template and start getting the bearing points in order. Once that is done, they start matching the table to the planer bed. Using the bed of the planer as a straight edge. Just like fitting a saddle to a lathe bed. There's a lot more to it than what I described, but give you a basic idea of how its done. And no, they don't use 20, 30, or 40 foot straight edges to scrape to. I have seen 10 and 12 foot straight edges in my past, never used them.
When I rebuild a machine that big I try to find a bigger machine to machine the smaller one. Ken is right on. One trick I use when rebuilding machines. When the ways are worn and it doesn't pay to send the machine out to be planed. I use the clearance surfaces right next to the way surfaces to indicate the from. As they were planed at the same set up when they planed. the ways and clearance.
Cash Masters rebuilds Mattison Grinders and he uses wire mic's. and short straight-edges working together .
Some big planers in Portland Oregon at Columbia Forge. Some other big machines and heat threat oven on rail road. Look at the cart and on the big VTL and see the big chain links and on the rail car, those are chains they cast in their foundry and anneal them before machining them. They pour the links connected to each other. They host scraping classes from time to time. They give the group a tour and it is really cool the way they pour those chains used in mines. They also make rock and car crushers. I'll have to see if they would host another class in 2019.
Back in my very young years, around 1977-1979, I used to work with a company in Dallas that had large Cincinnati planer. The man that ran that planer was already retired and still working. He started running planers in Saint Louis back in the 1940's. Planed a many lathe beds and other equipment for machine builders back in that time. This guy was so good, he could match plane bed sections on long bed machine tools like the trepanners I used to work on, that would have two, three, four or more bed sections! And when he was done, they would take and bolt the sections together and sight them with a transit and get then running within a couple of thousandths in 100 feet! Tell me that ant darn good.
Getting back to the OP's post above. Those guys back then, I guess they were millwrights that did the scraping fitting of machine tool components together? Anyways, between them and the guys that ran the planers, that planer operator could get the way profile so close, and I mean splitting hairs, that the millwrights had very little scraping to do to make that stuff fit perfectly. If you have the Connely book, look at the pictures of the planers being built and checked for alignment, impressive to me!
Machine was my baby in the seventies , planners and VBMs where my specialty in every shop I worked in . My final shop we had a very large planer mill , an upgrade from the tool type I cut my teeth on. Sure enjoyed those years working with those babies. Had a twelve ft dia stainless ring I had to cut a dove tail Oring groove for an electric pot oven done on there VBM . Inductotherm was famous for there ovens worldwide celmar my shop was a mile down there road.