slant bellhousings

I'm pretty sure you can look at your bellhousing from under the car and tell if it has the dual bolt pattern, as two of the four speed bolt holes should be outside of where your three speed transmission bolts on.

But I agree that a 67 three speed bellhousing would be a single pattern. Also that a 64-5 cast iron slant six four speed bellhousing would be the easiest to install. Problem would be finding one.

Finally, the 64-5 slant six and 273 four speeds had a 3.09 low gear. Later ones had 2.66. I'm pretty sure your present three speed low gear has a 3-something low gear ratio, while the four speed you plan to use would have a 2.66 low gear. Gonna be noticeably slower off the line with the taller 2.66 ratio, as I discovered years ago when I replaced the original 4-speed in my 65 Barracuda with a later one. And that was with a good running 340. Went from being fast off the line to being a dog off the line. Then I found out that my original 3.09 low gear gearset would fit into the later case, and so made the switch.