You want to make sure the rockers are riding on the top of the valve stem and not making contact with the retainers throughout the range of travel. If the rocker is making contact with the retainer you could cause all kinds of damage. Look over the spring retainers closely, the rocker arms should only make contact with the top of the valve:
http://www.97transam.com/harold/IMG_0274_pic.jpg
The retainers should show no wear marks that would show the rockers are making contact with the retainers.
Take ALL the pushrod and roll then across something flat ( piece of glass. mirror etc ) and make sure they are truely straight.
Your cam with 1.6 Rockers is 545 lift. If those springs aren't set right you are probably going into bind?????
Just because the springs are capable of handling 600 lift, doesn't really mean they automatically will. They need to be set up for that lift, and this is done with the install height. using certain locators/retainers and even cutting the pockets and each spring installed needs to be shimmed to the proper install weight.
If the springs are set too tight you could then be going into coil bind. You would need some pretty serious spring pressure to cause this type of problem, and that would more likely show up as lifters colapsing. Although that should be ruled out just yet?
To me, my first guess would be coil bind, this does sound like what is happeneing if you are breaking pushrods and pulling the studs out of the heads.
My advise, check the springs for coil bind,
Drop in a solid lifter, run each cylinder up to where each valve is at max lift and check for coil bind.