how to identify what power valve to use for my carb?

[340plot]

i bought the holley trick kit part number37-485 for my holley 750 d.p. manual choke. mechanical secondaries.

the power valve that i took out has no number marks on it, and the kit i got came with three different ones. none are identical at all.

and after starting to type this thread, i noticed a number on the top of the power valve that says 6.5 in a circle with a 3 on the line of the circle, and the numbers on the new ones do not match

the 3 power valves that came with the kit appear to be different
two have the numbers 665 or 995 on it, and the third one k805 ...... which one do i use?

 

[Shotgun]

To properly size a power valve, take a vacuum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vacuum reading in gear at idle and if the vacuum is below 12" divide that in half for proper size. Example 9" of vacuum in gear at idle will require a 4.5" power valve.