I have a recurved distributor with 16 degrees mechanical advance.
904 transmission, stock motor with headers and 2 barrel manifold and a Weber.
As I understand it, 30 degrees initial+mechanical is ideal.
A 10.5 or 11 vacuum can is needed to get up to the low 50's for economical cruising.

My research shows that:
Niehoff AL317 = GPSorensen VCC530 = Napa VC3029 = 8.5degrees


Neihoff AL346 = GPSorensen VCC535 = Napa VC3034 = 9.5 degrees


Neihoff AL348 = GPSorensen VCC537 = Napa VC3036 = 8.5 degrees


I've ordered a half dozen of these and they always come up 8.5 or 9.5 !
There must be differences in how much vacuum is needed to initiate the advance.
I really wanted to try an 11 can, but if I use one of these is it OK to recurve the distributor to have an init+mech of 35 or 36 degrees?

OK. I searched here and found that 32 is about the limit.

At the track I find that 28 to 30 degrees total timing make the car faster.
On the street, more advance gets better MPG, as long as you can keep the engine from pinging.
DD

Don't worry necessarily about making it to the low 50s with vacuum. A 9.5 can will give you 30+19 = 49, which will be very close for a Slant. I would run that for a while and see how it feels/performs/mpgs.

Lou

_________________
Home of Slant6-powered fun machines since 1988