When you are sitting the valves, are the lifters full of oil?
Has it been setting for a while?
Are you setting each cylinder one at a time, INT & EXH?
Please give as much information on your procedure so maybe we can figure out if it's your method or mechanical.
Snapping and bending parts is real sign or something to tight.
When the engine runs, does it run OK? just noisy, sluggish, hard starting?
What are the spec's of this engine? cam lift, rocker ratio, head type, etc...........
If you are not doing so already, I would strongly recommend using a poly-lock nut on each rocker stud. They are much easier to adjust lash with then a stock Chevrolet lock nut. I suspect you are running a 3/8" stud, it would be a chore to be breaking the 7/16" big block studs.
I would start with #1 cylinder, bring it up on the compression stroke ( both valves closed, piston TDC ) and set the lash to zero. Using poly-lock nuts, you can get zero lash by using your finger tips on the poly-lock. The pushrod should just touch the lifter and have a very slight drag. Even if the lifters are not full of oil, you can still find zero lash if you are careful on the adjustment, again, this is where poly-locks come in handy. At zero lash #1 cylinder, loosen the other 14 rockers so as to not bend or damage anything else. Rotate engine till the intake valve is fully open. You may have to rotate a few time till you are sure you are at full valve opening. Use the belts or a wrench to rotate ( pull the spark plugs ), NOT THE STARTER! Now, just as Evan said in the above post, check to see how much more the valve will open. Use a pry bar, screw driver, what ever it takes, to compress the rocker to it's maximum travel. If there is zero travel, or you can't even get to full lift, you have mismatched parts or poor machining. If you do have more travel, then set the other 14 rockers with zero lash and check each cylinder as you go. .060" or more is needed between each coil of the spring at full valve lift. If you cannot figure out how to pry the rocker to maximum opening, at least measure the gaps between the coils of the spring.
The engine should run with zero lash, it may be noisy but should run. If you are 100% sure you have hydraulic lifter, give each nut, 1/2 more turn from zero lash. It should run with no lifter clatter.
If you cannot get to fully open valves on #1, pull engine and take back to who ever built it!
