Edit:
Update below.
It is "possible", however........


How feasible is this?
I hate the 91 concave stock, aka, Meat Tenderizer.
I got a nice F stock armature a couple years back, the plan was to modify the arms, meaning, machine in the 2 extra notches for the slide catch, but looking at it, would it just be possible to machine off the rear arm catch?
Top parts in pic are blue/grey 91 stock parts salvaged from a stock that was on a F/A kaboom gun, middle is German A-3 stock, bottom F armature.
The pic shows the G3 stock with the 2 slots for the catch, F stock with only one.
Someone must have done this.
Bottom pic is one I found of some Norwegian dude in a really sandy place, appears it's an F stock on his, so.....
Any info appreciated.
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It looks like it might be possible, but likely two cuts were made deliberately for the increased recoil of the 308.  Not an expert on the recoil impulse, but I'd think it would add some concern about durability long term if you want to shoot it a lot.

When I was in my much dumber and eager days, I would have jumped right in and started as soon as the thought entered my confidence addled self.
That's a good point.

I don’t have anything useful to contribute beyond this is a really cool idea.

I prefer the A3F on MP5 clones, so a variation for the G3 would be perfect.  Too bad HK doesn’t make such an animal and I don’t think MKE makes F-pattern at all.

I’d agree that milling off the second lug is probably a bad idea.  Stock would likely sheer off the remaining one eventually and auto-collapse in an unexpected moment.  

At least the cut is the easier cut to make - If you take both to a machine shop or someone you might know with a mill, it isn't a complicated cut.

I used a 93/MP-5 single arm locking piece to fit in the 91 A-3 housing, see first pic, it worked with the other parts, release lever, ratchet nut, etc, BUT......it appears the MP-5 arms are wider.
I tried 2, the one that came with the MP-5 stock parts, the newer scalloped pattern, and an older non scalloped.
I can fit the housing onto both rifles I have, Greek SAR-8 and a PTR32, but the stock will not slide on the receivers.
I tried both, measured the arms and they are about .005" wider from outer face to the inside where they fit into the grooves on the receiver.
Compared them to a number of different stocks I have, and the MP-5 stock arms are made wider, meaning the MP-5 receiver is JUST a little more narrow than a 91/G3.
I originally thought of this for the PTR-32, sort of a "poor man's HK-32".
It'll work, but, it would mean removing a little material from both stock arms the full length.
On the brighter side, extended, the MP-5 stock does give the same LOP as the 91, thanks to the larger butt pad, but adds about an inch and a half closed, again, thanks to the larger pad.
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I would almost think if you know a good welder, you could cut the F-buttstock arms leaving a few inches of arm studs out the front- then cut the 91 A3 stock butt off simialrly weld the arms of the F and the 91A3 stock together - grind, finish welds off it should hold up good with a solid weld. They are steel.

Funny I did a 93 stock back in the day going the opposite way - modifying a surplus G3 A3 stock to 93 size backplate.

This is cool!  I used to own a G3A3 stock that had the arms milled to be 3 position.  Neat idea but the 3rd position was too short for me.  

I have thought about welding on a standard buttstock to create a flatter butt.  Not my skill set but I have a fried who can do it.

I have the parts for something like that. B&T buttplate I got with some other parts, and a set of NEW German G-3 A-3 stock arms.
I can't weld.....WELL, but I know someone who is able to.
If this doesn't work out, I might go that route.
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Someone pounded the butt plate off the old arms, probably "updated" it to something else.