wanting to tune my carb the old vac way but reading some info the vac advance tube on the base of the weber 34ich is not the place to put it.

my inlet manifold had a blanking plug which I can adapt, anyone know the thread size/pitch?

I guess it's blanked off unless you have servo assisted brakes.

3/8 BSF, same as sump plug

Yes, you want it to run on manifold pressure. The vac advance on the weber is ported (i.e. above the throttle) so it reads wrong at idle.

The vacuum is good but it doesn't tell you that much - if you can get access to a Wideband Lambda sensor then you can do some real tuning...

I keep meaning to put a vacuum guage in the cab permanently though as it helps combat my heavy right foot!