Lets start with the brakes first. See if your booster is working. You can do this by applying a firm pressure to the brake pedal with the engine OFF. While you have pressure applied to the pedal start the truck. The pedel should go down several inches. If it does, your booster is good. If it doesn't you need a booster.
Let's say the booster is good. Second, get some penetrating oil and spray down all the hardware in the drums (after you remove the drums & make sure you don't get any on the shoes). Then adjust the adjuster screws (the thing thats looks like a rod with a bunch of teeth on it at the top of the brake shoes). With the drums on, you do this from the inside of the dust plate. There should be either a slot or a little rectangular rubber boot there. If the rubber is there, use a screw driver to pry it off. You will be able to see the "teeth" of the adjuster thru the slot. adjust it by rotating the teeth (they should only go one way) till the drums just start to drag. reinstall the rubber boot if you took one off. If there isn't one there it's no big deal.
Next bleed the brakes. You will probably need to use a c-clamp or some other drvice to hold the pin on the fwd end of your proportioning valve in (thats the valve thats attached to the bracket thats attached to your master cylinder. There should be a couple of short brake lines goint from the MC to the valve, then on to the brakes). Then bleed the back right brake till you get clean fluid coming out. Then go to your back left, right front then left front. Make sure and keep an eye out on the fluid level of the Master cylinder while you do this. Don't run the reservior dry, or you'll be bleed ing your master cylinder and a whole lot more brake bleeding. Remove the clamp from the proportioning valve.
After doing all that try your brakes and see how they work.
As for you tranny. check and see if the modulating valve is blown. the easiest way is to remove it (Its a round cylinder at the back of the passangers side of the transmission. The vacume line hooks up to it). Turn it nipple side down and see if any transmission fluid comes out. If it does the valve diaphram is shot and you need a new one. If it's good reinstall it and make sure your getting vacume to it. Disconnect the line from the modulating valve and start the engine. Put your finger over the end of the rubber hose or hard line which ever is still going up to the intake manafold and see if its pulling a vacume. If it is good. If it isn't find out why. Either you have a ruptured rubber line or it isn't hooked up right. it should be "T"ed off from the same line that goes to your distributors vacume advance. You may also want to make sure the hard line isnt kinked or cracked also.
You also need to make sure your kick down cable is hooked up and adjusted right. Heres one link to adjusting the kick down cable
http://67-72chevytrucks.com/vboard/s...kickdown+cable. You can also check the search function and the FAQ sections.
After youve done all this lets us know if it helped or not. Happy wrenching.