Are all slant dip sticks the same length?Where to get one?

No. There are many different dipsticks and tubes for the Slant-6; they are not all the same. If you're using factory parts, you need a matched stick/tube/oil pan to get correct readings. If your dipstick tube is okeh and all you need is the stick and can't find an OE item, buy this and rob the stick out of it. Change your oil and filter, put in exactly four quarts of new oil, start the engine and run it long enough to fill the filter, park the truck on level ground and leave it sit for 20 minutes, then adjust the stick to a workable length, use it to check the oil, take note of the level, drill a hole and/or file a notch at the edge of the stick—that's your "one quart low" mark. Add another quart of oil to the engine, let it drip down for five minutes or so, check the oil again, and mark that level—that's your "full" mark.

Lower quality/lower price version of the universal stick here (probably still perfectly adequate).

The 2bbl "Super Six" setup runs a lot better than a 1bbl, assuming a carb in good condition. Better manners and better driveability, better performance and better economy. The setups have grown much more difficult to find and much more expensive than they used to be; this is a good deal. You don't really need the exhaust manifold (if yours is undamaged); the 1bbl and 2bbl exhaust manifolds are same. Do yourself a big favor and skip the junky factory choke; pick up a № 1232 electric choke kit and drop it on; you'll be a lot happier—they work better and they're adjustable.