1970 Pontiac Firebird cam and valve springs

Engine Mechanical problem
1970 Pontiac Firebird V8 Two Wheel Drive Manual

Hi,
My Car is a 1970 Trans am Ram Air lll
I had its engine overhauled
Piston, valves, and valve Springs changed with normal pontiac parts..
After around 100miles it broke one valve spring..
I changed it, after another 100miles it broke another one..I changed a set of valve springs..again the problem happened.

here is the info about its camshaft :
http://fiedlerh.home.att.net/PontiacCams.htm
the model is 9785744

Do you know what the problem is ?
And if I should use a special kind of Valve springs, can you please guide me exactly what should I buy?

Thanks