I dropped a Doug Nash 4+1  5 speed into my 1977 that had a 4 speed.  It was an easy bolt up to replace a 4 speed.  Sounds like your car came with a 403 and automatic,  you will need to change the brake/clutch hanger and find some clutch linkage.
There is already a hole on the firewall for the rod to go through, but you will have to make a big hole in your trans tunnel.   The cars that came with a floor shift have a small housing welded to the trans tunnel to support the gear shift boot.  Grab all this stuff off a donor in the junk yard.   Summit Racing sells the tranny.  I got the lowest 1st gear they had at the time  4.06.  Then I changed the rear diff gear to a 2.41.  This gave me over 20mpg on the highway. This setup is equal to the original 4 speed with a rear gear of 4.02 for acceleration. Hope this helps.