slant six cranks but won't start

If >100 psi compression in all cylinders, it should run. 120 psi is better. I recall the manual says ~140 psi for small blocks. Might be slightly less for slants.

Why don't they quote compression numbers more solidly? The gage you use can make a difference, both the volume displaced (should match spark plug exactly) and the Shraeder valve in the end (used as a check valve) takes some pressure drop which depends on the spring. Yours should be a special valve w/ white collar (denotes weak spring) made just for compression gages. The other factor is that the camshaft makes a big difference in readings, and people often change those.