If you remove the rudder
Putting a foam block insead of any kind of foot brace is easy. Positives include decreased cockpit volume where you do not need it (so easier to empty the water if swamped), more options for and more comfortable foot positions, allows for feet together paddling. Negatives include that, depending on the design, you may not be able to stretch your legs straight at any point for relaxation, harder to adjust fore/aft position (which is necessary to accomodate bare foot vs. footwear or different paddlers).
Foam requires that you have a strong bulkhead though - if yours is not welded in place but a glued foam block, I read that it is likely to fail. So you would need to design some sort of a brace to go behind it and that mounts on the existing rail screws. Much like they have on some WW kayaks with full foot board support that is attached to the hull where your foot rails would be rather than resting on the bulkhead (the WW boats often do not have a bulkhead in the front).
Kocho is right
about the weakness of the glued in foam bulkhead. Your older Arluk IV most likely has one. That was usual Necky practice back then. If you want to pull the rudder & go the foam block route, you could pull the existing foam bulkhead, replace it with one glassed in place, then install the appropriate thickness foam foot block. If you keep the rudder, definitely lose the POS stock Necky sliding controls. It cost me about $100 to swap mine out for the Seaward ones. For my boat, one of the Seaward bolt holes didn’t line up. Instead of patching & re-drilling another hole in my boat, I drilled and tapped a new hole in the pedal mounting track. It’s not hard, it makes a big difference in boat control, and you’re much less likely to pop a rudder cable with pedals that lock in place.