I'm having a real hard time finding out what the proper compressed valve spring height ought to be on a pair of #15 heads. If you think you might be able to help, PLEASE e-mail me. Thanks.

I'm having a real hard time finding out what the proper compressed valve spring height ought to be on a pair of #15 heads. If you think you might be able to help, PLEASE e-mail me. Thanks.

I'll take a shot at answering your question. On pages 6-77 and 6-78 of the 1968 Pontiac Service Manual, Valve Spring Pressure at Length figures are published, as well as valve lengths for the different heads.
A # 15 head is for '68 Pontiac 400. It takes an intake valve of 4.99" and an exhaust of 4.98" in overall length. Double Valve Springs are used in this head (and all '68 Pontiac heads). For Standard Valve Springs:
PSI @ Length
OUTER: 135-145 1.134"
59-65 1.586"
INNER: 99-105 1.114"
28-34 1.566"

Good Luck. Hope this helps.