My '55 TBird has it's original transmission.  My car came from the factory with a manual three speed transmission with O/D.  My transmission is a BW T86 top load three speed with a R10 overdrive unit.  I have owned a number of T85s over the years.  T85s are side load, much larger and much stronger.  They are not larger than an automatic.

There is some good info. on Ford Barn, regarding BW 259, T85, T86 transmissions and 55-57 Tbird transmission tunnels. According to CTCI, the BW 259 transmission (side-loader) was used in 55 Tbirds, not the T86. Tbird tunnels were enlarged to accommodate the T85 in 56. A T85 has the same mounting location as a T86, but the T85 tailshaft is longer. The 56-57 driveshafts are larger in diameter than the 55's. When installing a T85 in a 55 Tbird, the smaller diameter 55 driveshaft needs to be used, and it has to be shortened to accommodate the longer T85 tailshaft. 

I guess the million-dollar question, then, is a T5 bigger/smaller than a T85? 

A T85 is bigger than a Fordomatic?

I do not know if this is correct, but I have been told that '55 TBird's have the smallest transmission tunnel. This makes sense as '57 TBird manual transmission cars had T85s which are much larger than my T86.

Yep. When installing/removing a transmission from a 55-57 Tbird, you have two choices: Pull the engine or remove/install the transmission in pieces. 

I removed the transmission before removing the engine.  I slid the original three speed/OD back after removing the shifter and arms.  It would not go back far enough to drop the pilot shaft down, so I removed the bellhousing, and the transmission came out.  I was going to install the T-5 in reverse order, but the tailshaft housing is too large to go between the X frame and the tunnel sheetmetal.  Even after removing the engine and bolting everything up to re-install the engine, it would not clear the X frame without cutting the hole in the tunnel.  Joe-JDC

JDC

If you have a 1955 Tbird with a manual transmutation and O/D, you must remove the engine to remove the transmission. Even with the bolt-in cross member removed, the transmission will not move back far enough for the input shaft to clear the bell housing.  The Ford service manual says you can do this by loosening the bell housing which to me seemed worse than pull the motor.  It's a very tight fit.

Charlie, the shifter does not have to be installed first.  On a Mustang, or any car that came with the T-5 style transmission, you have a crossmember that bolts in and you can remove it to install the transmission.  The Thunderbird has a huge X frame right where the tailshaft and shifter housing need to be positioned.  The distance between the tunnel and the X frame is ~5 3/4", and the transmission tailshaft and housing without the shifter tower is ~6 3/4" thick.  So a hole needs to be cut from the tunnel to allow the transmission to slip past the X frame and for the shifter to be installed from the top after the transmission in in place.  The whole assembly would not slip between the X frame and the body of the tunnel without cutting the hole.  Kinda like the old saying "You can't put 10 lbs of potatoes in a 5 lb sack."  Joe-JDC

JDC

Every time I get into some new project on an old car it's the same.  You know you won't have time until _____ weeks go by so you spend that time buying 15 parts in hopes that one of them fit on the one day that you'll have to be able to get it all together.  It's either that or you spend all your time obsessing about how you're going to make something work and then you figure it out and find out that won't work at all and then something really simple (and cheaper than all the stuff you just bought) pops up...but now you need to wait a few more days for that stuff to come in the mail because you can't buy anything in person anymore for old cars.

I swapped the old 390 CFM Holley for a 600 CFM Holley this month.  Yes...I said this month.  O'Reilly's didn't have the carb base gaskets (no, seriously) so rather than drive around I just ordered them from Amazon.  The "2 day" shipping took 4 days, and when they (2 because I'm running a spacer) showed up, a genius in the had warehouse bent one of them to make it fit into the envelope they used, making a crease that essentially ruined that gasket.  I went to a speed shop that didn't even have the gasket I wanted.  Unreal.  
Just think what ELSE would have gone wrong had I done what I originally planned - to remove the intake and massage the carburetor ports.  

I've come to expect that when consensus says something "bolts right in" that something will need to be trimmed, at best.

Why does the shifter need to be installed first?  On my '88 Firebird (factory T5) and truck, I install the shifter after the transmission is installed.  I install the shifter from the top, through the floor.



Lawrenceville, GA