I agree with jamesc and 440Jim, more information would help. The intake manifold is one thing I would be interested in, as well as car weight with driver.

You netted about 20+ more hp. In analyzing the before and after numbers the real wildcard here in my opinion is; how good really were your 915 heads? I don't know what your car weighs, but if they went 150 mph they probably weren't terrible for o.e. iron heads, but I have a hunch the Victors and cam should have shown some more than they did unless the iron heads were very good.

But, I think to make substantial gains (8-12 mph) a good port job on the Victors and tweaking of other parts (intake, larger carburetor, and possibly different cam) need to be done. I am a little surprised as your stated compression ratio of 15.3 on that displacement is not common because it generally takes a fair amount of determination and effort to make it happen.

Are you sure the valve springs on the Victors are able to handle the roller lobes?