Regardless of whether you stick to the Dickson-style toolpost, or jump ship to the no doubt superior Multifix-style, with any system mounted on the topslide you are almost certain to face similar limitations.
I have about 20 toolholders for the Dickson toolpost on my Super7, and only about three of them have a base sufficiently thin to permit a 10mm tool to hit centre height – just.
It’s an exercise in futility though, as to achieve this the lower face of the (carefully selected) toolholder must then effectively rest on a scrupulously clean topslide surface – a position which is beyond the range of the usual height adjustment screw, and which negates at least one advantage of a QC toolpost.
I think you can find the dimensions of the the various Multifix-style toolholders on the pewetools.de website, if you want to check if they offer you any significant dimensional advantage, but I doubt they do.
I’m tempted to think that the Myford-Dickson toolpost gained its reputation as a “must-have” accessory for the Series 7 topslide when 1/4″ and 5/16″ HSS toolbits were the order of the day, and since then many of us (including myself) have been seduced by the toolholder capacity for 10/12mm carbide tools, and our reckless inference that these would therefore be a practical proposition!
I’ve seen, and considered, various solutions over the years – including skimming the topslide, skimming the toolholder bases, and making a wider toolpost (all of which have their drawbacks in my opinion) – but if you really want to utilise your existing 12mm tooling in a quick-change toolpost, perhaps the best idea might be to consider making a solid (lower!) block that fits in place of topslide, and mount the toolpost directly to that.
As an example, check out the one made by the “Optical Mechanic” on YouTube.
The topslide (and some 6mm tooling!) can then be refitted when actually needed – say for screwcutting and taper turning.
Clem.