Dennis,

  The driveshaft will have to be changed, as will the transmission
mounts. The hole may have to be enlarged for your aftermarket shifter
assembly, as the factory unit is not a direct "bolt in" affair. If your
car original came with the automatic, then unless your floor was cut for
the 4spd, you should be fine. Finally, as for being aware of anything
else, make sure you have access to alternate transportation for when you
find out your missing a bolt of two...................

Hope you find this helpful,

Sincerely,

Bill S.

Years ago, I did this swap on my 67 convertible.  I went from an M3 to a C4.
(Yes I kept the parts so I can put it back someday.)
1) No modifications were required to the tunnel as Bill S. also said.
However, before you install the torque converter, you will have to remove a
bronze pilot bushing from the hub of the crankshaft for the manual
transmission input shaft. I drilled mine out. Some people pack the hole with
grease and force it out by putting a shaft in the hole.
2) The number three crossmember is identical for both transmissions.
3) At the time, I took the driveshaft from the donor car. They were
identical and I didn't have to use it.
4) Wierd things you may encounter.
a) There is a heavy spring on the porkchop (aka pedal support) that is a
bear to hook up and remove. I think it was installed in one piece prior to
the installation of the long shaft steering column. If you choose to undo
the three bolts holding the bracket plate, be careful.
b) My 67 used a different gas pedal because of the kickdown cable. There may
be an extension you have to add to your gas pedal.

Regards, Tom Miller