Austin to Philly: Google maps shows 1659 miles by interstate - add probably 20% for secondary roads. That's one long a$$ trip in any vehicle and especially in an M38A1, but I can attest it is doable. Way back in 1984, when I was a lot younger and probably (?) stupider, I drove my M38 1500 miles with my new, and fortunately very patient, wife riding shotgun from northern Idaho to San Diego in the heat of summer. Took us the better part of five days with one repair stop for a leaking radiator filler neck. It didn't seem that crazy to me at the time - I had driven it 900 miles home from the Navy in Bremerton, WA to southern Idaho via some real back roads, then another 500+ mile trip from southern to northern Idaho to go to college. My driver vehicle in high school was a 20-year-old 1953 Ford pickup, so driving a rather slow, primitive vehicle was what I was used to, and I had learned how to troubleshoot and solve most basic ignition and fuel problems by the time I made these trips. In retrospect, I wasn't too bright because I didn't really know much at all about the mechanical condition of the chassis at the time. Now that my son and I have rebuilt the entire chassis and examined the condition of all the parts in painstaking detail, I think, "Holy crap - I drove all that way with it like this?!"
So, you are probably in a lot better position with your M38A1 than I was with my Jeep, but don't be fooled by the fact that "everything in there is brand new, or rebuilt new". That also means plenty of opportunity to have made a mistake or have a part with a defect. Plus, heading out on a long, hard haul on a rebuilt engine isn't easy on it. I'd put a few hundred miles on the Jeep locally, like the shakedown cruise we did in the Navy after an overhaul, before heading out on the long one. And then change the engine and gear oils to get rid of all the initial wear particles, and check all the steering, suspension and brake systems to verify everything is up to snuff. One missing cotter pin could ruin your day, and worse.