I converted my 65 barracuda from 3 on the tree to a 4-speed when I swapped the slant six for a smallblock. All the clutch linkage and z-bar, pedals etc cross right over. I cut a hole in the floor and mounted a floor hump (original, but there are repro ones I think now). At the time I just left the old column in place, and nobody seemed to notice that there was both a floor shifter AND a column shifter (unhooked).
Other things about this swap you didn't mention:
-Swap the /6 gas pedal and linkage for a V8 pedal and cable. there are nice repro early A pedal rubbers on ebay.
-Swap the /6 radiator for a V8 A-body radiator with the inlet on the other side. Any A-body should fit but early ones have a rounded look.
-The center link is different - the V8 one drops lower - but many people find they don't need it if they just shim the motor on its motor mounts to gain a little clearance.
-The motor you use will need an a-body pan, best early a, which has a cutout for the steering link. Same as above about clearance.
-Unless you are building a drag car, upgrade the torsion bar front springs - /6 cars had the smallest, and there are thicker ones for smallblock, bigblock, and then on up for more sporty handling. Running smallblock bars with a smallblock makes it 'normal' which is a little soft for modern standards. The bigblock bars on an early A with a smallblock are nice and crisp without making it feel like you're driving a lumber truck. (if you are building a drag car you'll want the /6 bars to help weight transfer, but it's not great for cornering)
-if your car has 9" front drum brakes PLEASE at least upgrade to 10" drums. Discs are better of course, but 10" are OK. 9" are just barely almost Ok if you keep them adjusted and maintained but if anything goes wrong you are screwed. Also recommend swapping to a dual master cylinder for safety. If the car has not been running you'll wind up replacing the MC and wheel cylinders anyway so might as well start that way. 10" drums also allow you to run the original 13" wheel covers which are so beautiful! (although nobody but me seems to care about them, so YMMV)
In general this is a very easy swap. Two details to get right to save hassle - the clutch, smallblock bellhousing, and gearbox input bearing have to fit each other. Likewise the shifter rods should be an original set with shifter. Piecing either of these sets of components together piece by piece is a PIA. Best scenario is to buy a transmission, bellhousing, and complete shifter that originally came together.
