Like always a very good advice, I'll try that
The vacuum advance is a part of the distributor? Mine doesn't have such a thing at all.
I keep forgetting you don't have a factory distributor. The vacuum advance would be evidenced by a vacuum line connected between the distributor (usually on the side next to the bottom edge of the distributor cap) and either the intake manifold or the carburetor. For distributors that have them, it's important to make all timing adjustments with the vacuum line disconnected and plugged.
Maybe I'll ask the shop to check my compression ratio as well if that isn't to complicated and expensive.
There's no way that I know of to measure compression ratio without removing the cylinder heads. Checking cylinder 'cranking pressure' with a gauge doesn't provide an accurate measurement because that measurement is heavlily influenced by the cam profile --- the "later" the intake valve closes, the less pressure will be - because compression can't begin until that valve closes. Speaking of, that's another "band aid" that's available: if you use an "adjustable" timing chain/gears you can retard cam timing "a little" to delay that intake valve closing event some and it just might be enough to keep the motor out of detonation if it's already "close". Retarding cam timing has other consequences though - it shifts the torque curve higher in the rpm range so the motor's going to have less torque at low rpm and also will need to turn to a higher rpm in order to make maximum power.
I just ran "numbers" for 670 heads on a standard 428 (4.12 bore, 4.00 stroke) and assumed a standard factory compressed gasket thicknetss of .042 with the pistons .020 "in the hole" at TDC - which is what Pontiacs "usually" are from the factory unless the block has been decked. I also assumed flat top pistons that have about 6 cc's worth of clearance volume in the valve reliefs. Factory specs for 670 heads state that their chambers have 72 cc's of clearance volume. However Pontiac heads have been known to vary from factory specs quite a bit in either direction, and also we don't know if your heads have ever been milled/surfaced, so the only way to know for sure would be to pull the heads and measure the chamber volumes directly. With all the "disclaimers" out of the way, that combination works out to 10.549:1 static compression using nominal factory measurements. That's probably too high for any pump gas we have available here in the States, I'm not sure about what you can get there in Austria. It might be possible to squeak by with a really nasty (long duration) cam and top notch cooling system, but I wouldn't try it myself. Just on numbers alone it's not worth it to push compression to the limits. On my 400/461 the difference between 9.4:1 and 10.5:1 is only worth about 14 peak horsepower - not worth it to me to take the risk of destroying the motor for just 14 HP.
To see the effect of chamber volume on static compression ratio, if we just add a measley 6 cc's of volume to each chamber 'somehow' (piston dish) that drops SCR to 9.96:1. If we triple the volume to a total of 18cc's (acheivable with off the shelf D-dished pistons) that makes SCR 9.44:1 - streetable on U.S. 93 octane if you're careful and have 'enough' cam.
Aluminum heads would be a different story. Aluminum doesn't retain heat like cast iron does so you can get away with more compression. 10.3:1 or maybe even 10.5:1 with aluminum heads is doable (given the right cam and a good cooling system).
Like with lots of things on cars, it's all a compromise and there's no hard and fast rule due to all the interelationships between the various components and the overall condition. A poorly planned/sloppily built motor might rattle like a load of loose bowling balls in a box car even at only 9:1 on 93 octane, while a very carefully built and meticulously maintained one might be ok at close to 10.1 if everything is always "dead on" and the fuel mixture/tune up always "perfect".
And... it's not that I'm an expert or anything on this topic, it's just that I spent BOATLOADS of time and effort researching it during the time I was planning and building my own 461. I was in a situation similar to yours. I had a set of original 1969 #722 Ram Air IV cylinder heads that I -really- wanted to use just for the cool factor, but in order to get the compression ratio down to something reasonable on a 461 I had to jump through a lot of hoops to do it, when it would have been "easier" to just get a good set of aluminum heads (which probably would have made more power than the IV's anyway). As it is though, I've got a 461 that makes fairly decent power that I can point to and say "yeah, it's just the factory 400 - nothing special" and MAYBE get away with it except with Pontiac-heads that know how to interpret all the various codes and numbers - at least until I start it up