Gear choice decisions

Yes and no;
As for handling abuse;
the first three gears are pretty much bullet-proof, at least as far as I have experienced with my modest 367 power(maybe 400hp). However, as mentioned in an earlier comment, that overdrive gear, I always say, is fragile, mainly cuz I regularly blew them up. However that is probably not fair, cuz when I blew the overdrive gears up, it was every time cuz I failed to back out of the throttle on the 3>4 shift, or was late to back off. Driven normally, they all did well for months to years, even with my heavy factory flywheel...
and
several other guys here on FABO are running them and not a one has mentioned the same failure.

As for dimensions;
the boxes themselves are a direct fit.
However, this box has a couple of quirks;
1) is that the front bearing retainer is 5.125 versus 4.35 on the standard. Thus something has to be modified or swapped, to facilitate this. Parts are available, and usually cheap enough.
2) another is that the box has been factory modified in the area of the front cluster pin, in an effort to reduce so called rattle. They say, and I have not experienced it with my modest 367, that this front hole can, over time, become oval. The prevention is to just bush it back to cast-iron dimension.
3) the box is available in several flavors, and one of them is the LD mainshaft, that fits the A904 yoke. I have no experience with that item, cuz my first box had the 727 sized shaft which I carried into subsequent gearsets. One of the problems with the LD shaft, is that the associated A904 yoke carries the smaller 7260 U-joint, whereas the A727 carries the larger 7290 Ujoint. and you can get a 1350 U-joint slip-yoke for the larger shaft, if you think you need it.
4) I found a long-tail big-shaft version at the scrapyard, which I bought for parts. In the end I installed it, and used the rear shifter mount to move my shifter back between the buckets, and closer to my hip/thigh, and high enough to install the Top mounting bolt from inside the cab. Then I installed a short-stick shifter, and fabricated new rods using pipe-steel. With the shifter now falling to hand, I haven't missed a shift since 2004.
5) the main box is available in both C.I. and aluminum; and some/perhaps most overdrive gear sets can be installed in either box, with minor grinding sometimes required. . and man, that C.I. case is a porker compared to the alloy. Tails are available in aluminum as well, as are the side-covers. As to retainers IDK, mine were all iron.
6) lessee what else? I think that's it!