The biggest thing you’ll want to look at is any flat spotting on the bottom of the chassis, clear signs of damage from contact, and any weld cracks/repairs. A chassis that old is bound to have a scraped up bottom which is normal but you don’t want any flat spots. Look for any cracks in the welds and possible repairs (usually you can pick these out as they are painted over which won’t match the finish of the original powder coat). Damage from contact will typically show up in a bent steering shaft (turn the wheel and see if it wobbles), replacement tie rods (see if they match and if they don’t than ask why), replacement spindles, etc.
Look very closely at the seat mounting struts for signs of damage/repair. It’s fairly normal to see repairs in this area, you just want to make sure if there is a repair than it is repaired properly.
In a perfect world you’d want to measure if the chassis is straight but this is not always easy to do without a flat table to put it on.