Hey,
I drive my '66 with totally stock running gear 65 mph on the highway EVERY time I get on the expressway. There are times where I keep it on 70. The drivetrain is most comfortable at 60.
This is not a muscle truck (250 inline-6, 3 speed column shift, 3.73 rear end and 235-70R15). I can keep this speed (and these rpms) because the 250 can spin at 3k all day with no problems. This is what I love about the newer classics.
To get your truck up to these speeds, and even more importantly to safely stop it from those speeds, you will have to do extensive upgrades. More than what you have listed here. To get a steady 70 mph cruising speed, you'll need to upgrade your gearing (either your rear end, your tranny or both). Taller tires will help too. A stock built 49 Chevy is not safe traveling 70. Plan on an IFS swap, with front disk brakes. Won't be much more trouble than the upgrades you listed, I'm sure. And your stopping ability and handling will increase tenfold.
Don't know how much later of a six you're thinking of. A later 261 would probably be the easiest power upgrade. If you go with the 7 bolt design (250/292), then you'll have to work out something for the motor mounts and change to an open driveline (and rear end). A 350 swap would probably be cheaper and yield much more power. Hope this helps.
-ftyler
