I don't think I would try to tow 5500# at " highway speeds " with a flatty !!!!!
And they only had a 81" WB back then and 50" WMS just for something to think about.
That and 60 gross h.p. , sounds like the perfect highway tow rig !
And let's not forget their HUGE 9.5" drum brakes... On all 4 corners. I can remember when upgrading to 11" drum brakes was a big deal.
Its hard to have too much stopping power. Having said that, any improvement over your current situation is always welcome.