early 904 question

I would do none of this

Use later stuff. You may very well find "a deal" on a later engine, and you'd have to build a bushing for the early trans snout

66's were not push button, so that trans is not "native" to anything you are dealing with. Besides, push button stuff is getting more difficult to find each year, and you'd either have to find a good shaft with "the boot" or have one (expensive) built with a built-in slip joint

It should NOT be tough to find a SB either 904 or 727 fairly cheap locally.

Drill the crank, if you want to use the 273, or I think some ATF people sell converters with the smaller snout, but I would drill the crank for the reasons stated.

If you go 4 speed, CAREFULLY check the crank DEPTH as some were not drilled deep enough Many gag, but the usual solution "back then" was to cut 3/8" or so off the 4 speed shaft.

I've REALLY grown to like a TF and I used to LOVE 4 speeds. But my legs no longer "support" heavy clutches, so I went that way "by design." I found a manual/ reverse shift body and just LOVE it.

(A side note, if this is a winter driver, using a manual VB would be GREAT in the winter, as you can lock it in 2nd or even 3rd to "kill torque" for those slick starts.)

WHAT engine is coming out? I believe if it's either a 6 or 273, and you get a 904, you can use your existing driveshaft, I'm guessing this car was not a stick?