FWIW,right after I got my '62 and not long before I put the first 350 into it,I drove it to Carson City,cruising at about 70 mph. I guessed at the time that I was screaming the motor,but it felt okay,and with rattles,squeaks,wind noise,gear noise and a marginal exhaust system,I couldn't hear anything. It made it to Carson empty and came home with a full load of furniture and appliances without a problem. My number cruncher shows I put about 700 miles on it running 4,200 rpms. I ran it around town for about a month afterwards with no problems,decided to go to SLC in it to pick up a 350 for it. I was in less of a hurry this time so I kept it around 60 mph. Made it about 50 miles before it dropped a cylinder,and developed a REAL knock. I thought it was a dropped valve,but found later it was a burned piston and metal contamination from that ruined a rod bearing and journal.
So-maybe you can go 75,but not for long and expect it to live. (or 60,for that matter...)
Speed
