With the 49's stock gearing, the motor won't like to run more than 55 (assuming it's a car or 1/2 ton PU). Upgrading the rear end gear and keeping everything else stock (but do add that swaybar) will work just fine and keep the value of the truck, as well as be easy. You can add disc brakes if you want but the drums work fine as long as you keep them well maintained- adjust them every time you change the oil and change the fluid every 6 months, and rebuild the wheel and master cylinder every 3-5 years. Make sure the emergency brake works.
The stock 216 babbit pounder will last a good long time as long as you don't abuse it. Don't overrev it (3200 rpm is tops), change the oil every 1000 miles, use good oil, keep it tuned up, and it will last a long time and be reliable. You can limp around for years with a knocking bearing on one because they are so thick. If your stock engine is in bad shape a later ('54-'62) 235 or 261 will bolt right in- you don't have to change motor mounts (you may have to drill a hole or two on the engine itself). The later engines have full pressure lube and better head flow for more power. There are speed parts for the old engines.
I am swapping a '57 block into my truck along with a 3.55 rear end (I have a 4 speed too) and I expect 70 to be perfectly reasonable. I still have the 216 in it right now, and other than heavy wear on the bearings (oil pressure is getting low) runs great and gets up to 55 just fine. It runs silky smooth and drives really nice, just a bit slow.
You will want to check the condition of the king pins and tie rod ends because most people don't lube them enough and they get loose. They hold up well if you lube them religiously though.
