Im in the process of putting together a truck, "Frankenstein" is the name it acquired.. lol. Anyhow the rolling chassis I got was from a fire co water truck. No body with the chassis unfortunately. But I also noticed that there were no shocks on the rear, nor a crossmember or frame mount to mount then to?? The brackets are still on the differential housing tho. My question is, Whats supposed to be there? Or is this common?
Wrecking yard. Find a bad frame on a half ton and torch the crossmember out with the frame rails and rivets in tact after you measure the mounting location. Take it home and get the rivets out. You'll likely find that the rivets are a bit larger than 3/8. Try a 10mm or 7/16 in the hole. Mark your hole location and drill! Install with grade 8 bolts. Some will just measure an existing truck and weld some channel or angle iron in its place and attach some shock mounts. Not recommended, though. Always bolt instead of weld. Welds create stress and crack.
Back in the day that I worked in truck shops we did much frame work. Heavy truck frames are often bolted together rather than riveted. Many frames and replacement frame rails had a caution sticker that ominously said "Do not drill flanges or weld to rails," a caution I have observed since. In retrofitting my 1-ton for rear shocks I made brackets out of spare spring leafs bent at a 90 with a couple of bolt holes torched in them. Then I found a convenient bolt hole already in the frame and bolted on the bracket. Drilling bolt holes in truck frames is not fun unless you have a magnetic drill press for the purpose. On aluminum frames we used a clamp affair to hold the drill. One caution is to make the bracket to take a shock with an eye on top rather than a stud. Your choice of shocks will be much greater. The AD 1-ton RPO for shocks used a forged bracket that riveted or bolted to the frame and used shocks with eyes top and bottom. Keep an eye out for that bracket.