The Valsan conversion replaced #1 and #3 with JT8D-217s and left #2 a -7 without a reverser. Some of them have since been upgraded to -219s and I know one has a -17 in the center. I can not imagine how that plane must go. Dee Howard put RR Tays on all three positions and there is a lot of new pneumatic plumbing in the back end. And the cockpit was updated.
Vmo was .90, I've been up to .885. Ours were limited to 350 kts, light weights could go for 400. A great plane for what it was designed to do, go 200-1500 miles between smaller airports when fuel was 10 cents a gallon. Some of the VIP planes have aux tanks to the point almost half the TO weight is fuel, pushing 80,000 pounds of Jet A. I'd hate to sign that reciept at today's rates.