What trans? You can get
1) a bellhousing,
2) a crank adaptor and bellhousing
3) or get a combination block adaptor and crank adaptor.
Ford has used all three methods in the past to fit proprierty gearboxes
The I6 block problem is very small. The only issue is that you may have to grind a very small amount away with a die grinder for stock Chev starter or AOD motors on the small bellhousing I6. If you use a Magnaquench, Denso or Modular V8 or aftermarket FE V8 starter motor, you can do it with no grinding at all. There is no bolt overlap issue unless you are putting a tiny Ford Pinto or 2.77 bellhousing on a bigger Aussie block, an unlikely set-up.
Bell housing swaps
can be expensive unless you have a very specific package. The really delightfull key is that Ford made a C3 to Ford 200 gearbox bellhousing from 1979 to 1981, and if you track that down, you can get a 1996 to 2000 4 or 5 speed Explorer/Ranger gearbox to hook up. After then, Ford went to a one piece transmission. So there is a really good all factory option there for some one to, um, explore.
Crank adaptor and bellhousing kits have been done since 1957. Early Borg Warner 35 gearboxes used on British and German Fords have used a special adaptor. Examples were Pinto OHC's, Kent Pushrod engines, the Zephyr 2.6 Liter I6, and the myriads of 2, 2.3, 2.4, 2.6, 2.8, and 2.9 V6's your US 4.0 60 degree V6 is based on. The Pinto 2000 engine used one for three years before the C4 and C3 automatics arrived. The latest Colonge V6 in the Explorer still uses a crank spacer.
Crank adaptor and blockplates have been used for the last 40 years. Flathead Ford boys have been bolting C4 gearboxes to things like there 239 Mercury lead sleads, and there are still block plate kits around to bolt an AOD to a flathead with no mods to the block. Ford has actually done it from 1958 to date on there six cylinder cars like the Bristish Zephyr six and the V6 Capri., So did the early XP 200 cube Australian I6's briefly from 1964 to 1965.
Here is a 1970 BW 35 Pinto crank spacer along side a special 289 Windsor to Chevy V8 crank adaptor (part of a THM 700 to Windsor V8 combination I've been working on).
It really doesn't matter if its a 1966 US Ford 200 block
or a 1966 Aussie Ford 200 block
or a 1987 Aussie Ford Crossflow block
Any Ford or Chevy or Buick Olds Pontiac gearbox will be able to be bolted to the I6. But its just for X-flow sixes, so you'd be better off making your own!
Fords ancient C3 (born 1974, and still related the the 5 speed auto in the latest Explorer), Borg Warner's even older 35 (born 1957, and still used on Fords in 1993!), Fords 1980 onwards AOD, they are all doable.
Pick of the bunch is the 1985 to date 4 and 5 speed gearboxes related to the German designed, French made C3 and A4LD, dating back to 1974. Using the 1979 to 1981 C3 bellhousing, you can fit it anywhere..1960 to 1962 round bodies should be a cinch with the smaller C3 style bellhousing. Here is a Cologne V6 verses AOD, I think you'll appreciate the compact nature of these under-rated gearboxes.
Its so much smaller than the AOD, and parts are everywhere
Here was one posted a while back. Its the French made C3, with US bellhousing. Same as every Pinto C3 since 1974-1980, every Mustang II C3, and but this one has a special flexplate, converter and bellhousing and seperator plate which can be fitted to the A4LD and later lock-up clutch gearboxes with not too much effort.
Compare it to the stock Pinto C3.
The latest 4.0 V6 item is still based on the French Bordeux C3, but has extra gearsets like the A4LD, and a has one has a one piece case, sort of like what Ford USA has done with the ZF based six speed 6H. They take good, sound German know-how, and wrap it up in a one piece case that everyone thinks cannot be retrofitted to an earlier I6.