Who offers that "bent" oil filler extension? I've only seen the straight ones, it looks like yours would be easier to add oil without spillage.
Gene Berg linkage can be adapted to just about any carbs on the planet. It's intended to be mounted to the shroud using three stock m6 bolt locations; with the centermount only two remain and they're relocated so you'd need to work that out but it shouldn't be difficult. Obviously it's vitally important that the shroud be solidly mounted, so additional bracing off of the case and/or alternator may be needed to keep the linkage from shifting.
Since (like the crossbar style) it pushes down on the throttle levers rather than pushing or pulling them horizontally, it's unaffected by engine-width growth.
An issue I've encountered with the crossbar linkage is that since the cable lever isn't centered on the bar there's more twist on the RH side, and any play in the pivot points also reduces travel at the RH carb - typically you need a slightly shorter lever on the RH carb than on the LH to compensate. The Berg linkage almost totally avoids these effects, and usually all that's needed is to bend the LH upper linkage arm to the side a little to shorten its effective length.
With very few exceptions, all you need to do when switching to different carbs is change the down-rods. Berg offers several varieties but they're easy to make from ¼" bar stock using an NF die. Both ends are RH thread so fine adjustments by simply twirling the rods isn't feasible, you need to unbolt one end and adjust the length in full-turn increments, but once initial setup is done they rarely need tweaking.
It costs about $200 new but it could be the last linkage you ever buy. Would not be too hard to make one yourself that incorporates the same principles - it looks like they used a bedframe rail to prototype the design
Another thought - could be used on either the crossbar or Berg-style linkage - would be to use a longer throttle cable routed through a Bowden tube to bring it over the shroud and back around to a bracket off the alternator housing so it could pull on an upward-pointed lever moved to dead center. On my mid-engine car I did something similar (of course it needed a
shorter cable and it didn't need to turn 180°) which worked well.