DaveBPT,
I agree with the previous posters that the T85 transmission is stronger than the T86.
I don't believe however that the T85 was used in the 1956 passenger car behind the 312.
I am building a T85 to replace the Fordomatic in my Victoria hardtop and I have found very little to support that they were. It appears that the 56 Police Interceptors and station wagons may have, as the listing for the standard transmission crossmember for 56 models excludes PI and SW applications and lists another number for them. That may explain why the 56/57 parts manuals list A parts (fits passenger car) that would only fit a T85 and not a T86. They were for the PI and SW vehicles.
I've attached some info I put together after measuring some transmissions in my shop. It shows you really need a 57 tailhousing get the mounting holes within 3/4" of the crossmember. Then it just requires a flat plate and some drilling.
If you do go with a T85 you need to pull the side cover and check the low/reverse gear for broken teeth. T85's do not have a synchro low gear so the low/reverse gear takes quite a bit of punishment until you get in some practice double clutching.
So if you want replace what was in your car from the factory I believe it should be a T86, or even possibly a BW 259, but if acceleration and speed are in the mix, it really should be a T85.
Don