Yeah, I hear ya on that. And "custom" steering is always a can-o-worms on Broncos.
Can you say, "snowball" and cascade effect?!!!!;D
The only bolt-on boxes I'm aware of are the stock originals and these. Others, like the 4x4x2, old Scout II boxes, and maybe even the Nissan conversion boxes are almost bolt-ons, with maybe only one hole to drill, but that's only close and not really a bolt-on.
We sold rebuilt and quickened stock boxes, but like so often over the years, the cores dried up again and so they're just not readily available like they were up to about a year and a half ago.
You might still find a vendor with some, or as mentioned get lucky and find one being sold by a member here or on Craigslist.
But the bottom line for me was overall strength, rather than originality. The old stock boxes did have their weak links that the new ones don't share. Yes, they had fully surrounded holes, but the cases and sector shafts (two pretty important things in gear boxes!) were still the weak links. Just not around the bolt holes.
So stock is not really the best choice for 35's in my opinion.
For years the Jamboree's on the Rubicon barred most EB's from participating in some events because the steering boxes were prone to breakage, and that was not something their "shop" could fix as easily as they could Jeep axles and such. No spares just laying around either.
I know GDDYAP here would have said the stock ones were fine too, up until last year. But on his last wheeling trip he split the case on his '73's original. With 35's.
On the same trip, another friend split the case or something similar on his stock '77 box too.
While one was pulled over to help the other, someone cruised by and said "hey, is that fluid coming out of the steering box?" about the OTHER Bronco! So two down in one fell swoop.
You may not be planning any wheeling with yours, but tires can still put good force all on their own.
The case at least on these aftermarket replacements is beefier and has deeper reinforcing webs than the original Saginaw boxes did. Plus our experience over the years with aftermarket boxes is that the sector shafts are at least a little stronger than stock, since I've never heard (yet) of one breaking.
So there's all that to promote the newer ones, even with their wobbled out mounting hole.
Paul