Hi ZeroR2,
Something to at least be aware of, if you aren't already, before drilling those holes in your board is this:
https://www.slingshotsports.com/M6-Bolt-AdapterI haven't used them myself, but they could be an option, at least temporarily. Also I know Grant on here tried them and had them break, which could be ugly. He has since gone away from using them. I suspect he may chime in here again, as he knows this stuff inside out. He talks about it here:
http://www.iwindsurf.com/forums/viewtopic.php?p=338980The other option I've heard of, on the iwindsurf forum I think, is people drilling out the holes, then getting some stainless washer/shims custom made that drop into the holes returning them to 6mm to allow the use of other manufacturers' fins, foils and wings. For fins, I suspect you might even be able to get away just using 6mm bolts in the 10mm holes without doing this provided you have good wide washers on top as most, if not all of the load force, is on the box and not on the bolts themselves.
I am not aware of any adapter block to fill in the the gap between a Slingshot Tuttle connector and a deep Tuttle box. Here's the thing though, I ride a first generation Wizard 125L. I took measurements inside its box, and it seems to have a deep Tuttle, while the Slingshot Tuttle head is clearly a regular Tuttle. This leads me to believe I have a gap in the foil box. With the collar on the foil's connector, and the contact area inside the box, the connection is rock solid. It seems that the 105L version of the board, and the subsequent year model of the 125L have regular Tuttle boxes.
My previous board was a Starboard slalom board with a Powerplate, and it was terrific. It's a bit more work to install and keep greased up to prevent corrosion, but it's what allowed me to get into windfoiling. Given you have a dedicated foil board I'd try to avoid the Powerplate, as it's expensive and does add a considerable amount of weight, otherwise they're a great option.
One last thing to consider is whether you actually really will be fin sailing the board at all. I bought my Starboard thinking I'd likely use it for both. Once I had my first session windfoiling on it, that was it, I put the fin away and never looked back. The foil got me going in far lighter wind and kept me going down to a 3.7, which would have meant a 4.2 with a fin. There's no way I'd ever fin sail a 122L board with a 4.2!
Have a look here if you haven't already, it seems to be the best English language resource for foiling info for questions like yours, I'm sure there are already several relevant threads there and on iwindsurf's forums.
EDIT: I meant to put this here, Telus' post reminded me:
https://www.seabreeze.com.au/forums/Windsurfing/FoilingSeattle has quite a few windfoilers too who I'm sure could weigh in as well, those guys don't hesitate to hack their gear to make it work:
https://groups.io/g/nw-windtalk/topicsGood luck!
Adam