Nothing about the SCELBI is straight forward. The cutouts on the back of the chassis for the 78S type connectors is another example of this.
I puzzled over how to cleanly accomplish this for quite a few hours. Note that the power connector is a 86CP4 plug, so we are talking about 14 holes for the I/O ports and 1 hole for the power connector for a total of 15 holes.
After much research, I finally happened upon a tool that was expressly designed to punch out these holes. The solution is a tool made by Greenlee called a radio chassis punch. The size you need for 78S type sockets is the type 732, sized 1 11/64″. They appear to sell pretty frequently for under $50 on ebay, but I happened to find a NOS one on a nearby distributers shelf for under $25.
Here are some example holes made by the punch. There is a 78S socket in one hole and the punch in another. You need to drill a 1/2 inch pilot hole before you can punch out the final hole with this tool.
One thing I did to make punching out my chassis more accurate, was to create a pattern based on a 12″ by 3″ chassis back panel. I determined that the spacing between the 1 1/4″ diameter connectors is only 1/8″, so your layout has to be pretty accurate. My BUD AC413 chassis is only 3″ high, so you will have to adjust vertical dimensions a bit for a SCELBI 3.5″ high chassis.
The next step is to install all those sockets and wire them up.