RB -- A lot of good input above. I think you can boil this conversation down to this (in my opinion ...):
Our universe (the Old Truck-o-Verse) is one of compromise/trade-offs. In your case, if you want to keep the wheels, you have to accept the limited tire availability... and the speed restrictions that go along with that.
In my opinion, these old trucks really have no reason or need to be in the 70 MPH plus speed range. Granted, even the mildest of mods (like mine, for example -- just an engine swap to a 261 and a 4.11 ratio upgrade in the rear axle) we do just to be ability to drive in modern traffic) give us the able to not only stay up with traffic and not be rolling road blocks, but also the ability to hit top speeds above 70 mph. But as stated above and elsewhere by smarter people than me ... just because you *can* doesn't mean you *should.* Next time you're above 65 MPH, think about those bias plys ... those nearly 90-year old rivets holding the wheel together ... that solid steel rod aimed right at your chest .... See what I mean? So yes, your truck will probably be able to go above 70 mph. but you won't like it.
My truck (I'm on newish 17-inch bias plys, too) can do 72 mph. It was NOT an enjoyable experience (For the ODSS members, I was on MD 32 going by Ft Meade trying to keep up with Sweet going to Bill Marlow's). I find a more sedate 60-63 or so is enjoyable for me AND the truck.
I find that cruising at about 62 MPH is just fine. Driving an 18 (22, actually) wheeler, hauling heavy equipment got me used to ignoring the impatient drivers stacking up behind me because ... gasp!! ... I was only doing the speed limit. They'll be fine until they can safely pass you and live their hurried little lives.
IMO, those who desire to cruise around at 70-80 MPH all day (even if you've upgraded to a v8, automatic tranny, radials, IFS, disc brakes, 3-pt harness, yadda yadda yadda) ... you're in the wrong vehicle. Go get a Corvette.
Just sayin'
