Contact GTS here in the forum for custom cabs, his work is superb. I have not converted a Mark IV combo, but I did convert my old DC-2 combo to a headshell. The process is very easy if you have the correct shell and reverb tank (combo tank may not work without a kit-the combo tanks require a metal plate/cover to be correctly installed in a headshell, you can get kits direct from Mesa). You may also need to get a different mounting bracket for the fan, I am not familiar with the mounting brackets on the combos. I can tell you this much about the headshells; my Mark I reissue, Mark IV, and DC-2 shells would all interchange with no problem. Aside from the minor differences in the Mark IV shell (footswitch cutouts on the rear, velcro grille & lower front cutout for preamp tube access) they were all identical. If you use a shell other than a Mark IV shell you may not get to your preamp tubes as quickly or easily and you will not be able to mount the footswitch in the rear. Hope this points you in the right direction.
