I'd been looking all over the place for a How-To on this, and I finally wound up here. I registered just to give you a huge Thank You! (But I'll certainly look around given Fountain Pens are quickly becoming a hobby of mine.)
I purchased a brand new Noodler's Flex Ahab just to try and mount the G nib, since I love the versatility of the Nib but I'm not a big fan of Dip pens. I managed to successfully heat set my Ebonite to fit the Nib (although I didn't know it for sure until I used my Dremel tool to shape the ebonite to fit the Nib). I wasn't sure what to do about the nib still not sitting flat however until I saw your post here -- and I figured I'd give it a try.
Long story short, it only took a few minutes with the dremel tool to get the G nib to fit properly - I spent more time recutting some of the shallow fins with a X-acto knife. By cutting just a tiny more material off the back I managed to fit the nib flat - so for me it's working perfectly with no tape, with the friction feed secure yet easy to slide in and out for cleaning / nib replacement. I can upload some pics if desired, but essentially it's what you already have. I did put the nib on top of the ebonite first though and mentally mark exactly where the nib touched as the area to specifically reshape.
This was after squeezing the heck out of it trying to get it to get it to fit via heat setting it alone, which didn't work by itself (hey, I thought I might get lucky!), but at least it meant I only had a little grinding and recutting the fins to go before everything worked.
While I mainly ink illustrations/comics with a brush, I wanted a pen for areas I needed more accuracy (and easier crosshatching than the brush) and this fit the bill perfectly. As I said, Dip pens aren't my thing - Just too finicky and having to re-ink constantly interrupts my work too much. I've only spent an hour or so with my new G nib fountain pen, but other than some railroading when I draw too fast or too heavy a line (as you already mentioned) it's working perfectly. I'm in Heaven.
I'm currently using Noodler's Heart of Darkness ink which works perfectly, although I have a number of other sample inks to try, as well as a bottle of Sailor's Kira-Guro that I use for my Kuretake Sable Hair brush pen. (As far as Waterproof Blacks go that is).
I can see myself doing this modification to several Ahab's I'll get in the near future though to hold my favorite inks... one for Noodler's Apache Sunset, Diamine Red Dragon, Pilot Iroshizuku Kon-Peki, etc.