RE: switching from a 3 speed to a 4 speed?
go to keislerauto.com they have what you need to convert to a 5 speed. it will probably run you a between 3 and 4 thousand since you already
have a stick in the car.
as far as a 4 speed, a good rebuilt one from a reputable place will run you $800.00 plus you will have to change out everything but the pedals and clutch linkage.
i am getting ready to convert my 73 with a 440 from an auto to a 5 speed. gonna run me about 5 thousand. yikes
good luck