Instead of putting in v43 scsi.device/FFS, could 64bit support be added to a new revision of v40? i.e., what I'm wondering with this line of inquiry is whether present day coding/compiling methods could achieve 64bit functionality in a smaller package than the ~1999 v43. Doesn't v43 also have all that NSD stuff (that I never really understood)? I would imagine that uses up some space and wouldn't be necessary for 3.1.
It should be possible to add 64 bit support to the V40 version of scsi.device and the FFS, but then again, it has been done already as well as it could be done. In my book, if Heinz Wrobel rose the challenge, the job is done and need not be tackled again. Possibly for the next few decades.
Both scsi.device and the FFS are written entirely in hand-crafted assembly language (with the exception of the Fast SCSI variant of scsi.device, which contains 'C' code and firmware bytecode for the SCSI microcontroller). This is, in my opinion, some of the most complex code in the entire Amiga operating system. And it was originally written by Randell Jesup, to whom I ever so humbly doff my cap. I forgot: the FFS and scsi.device are built on top of code written by Steve Beats.
While retrofitting 64 bit support is clearly possible, it's a really challenging task. Arguably, FFS may be easier to upgrade, but the real challenge is in making scsi.device work. Not only do you have to safely support the 64 bit I/O command set (NSD), you also have to support the gaggle of different hardware platforms which scsi.device works with.
The scsi.device still contains code to support ST and XT devices (as commonly used in 1987/1988 such as in the A590; I think this code is "dormant", though), the WD SCSI hardware (A590, A3000, etc.), the IDE hardware in the A1200, A4000 and A4000T, as well as the LSI Fast SCSI hardware used by the A4000T. The V43 scsi.device upgraded the ATA support as well as the Fast SCSI support, which along with the 64 bit I/O support accounts for the increased size of the driver binary.
If you ask me (well, you did ask me, didn't you?), this is the kind of code I'd rather find an excellent excuse not to modify. That level of complexity takes a very special kind of person to approach it. I had my brush with glory at this level of complexity with the FFS reimplementation which is part of AmigaOS4 and MorphOS. While the driving force behind this project was my own curiousity, I would not want to repeat the exercise. In particular not if the exercise involves writing faultless 68000 assembly language. This screams to me "run away!", in Graham Chapman's voice

Bottom line is, I believe that the V43 scsi.device and FFS are the best possible solutions for the problem at hand, although they create problems with ROM space.