I think mine is a 108N. Your 68 400 is basically the same engine as my 68 428, except internal dimensions of stroke, bore and camshaft. It's also a pretty tight fit between the headers and the pan. One can rotate it in the mounting block to get the thing kinda centered. No room to take the cable off or on mine with it mounted so I have to attach the cable before I install the starter. Don't have to take off the headers though. If you do get one, after you know where it will be indexed on the mounting block, remove the starter to mounting block bolts and add some thread lock. Mine came loose from the mounting block a couple years ago, been tight ever since.

Wouldn't hurt to look at rebuilding your old one. Take it apart and see if it's in good shape other than the solenoid. Used to be able to buy replacement solenoids for them, don't know if that is still the case. I took one apart and just flipped the contacts over, worked great after that. The originals seemed to be affected by heat, either the starter itself or the S wire from the switch. Sometimes the starter got so hot it would drag. Sometimes the large purple start wire from the switch through the heat tube on the rear of the block, got so hot nothing would happen when you turned the key.. Most often that would be after it sat for about 10 minutes after shut down and the wires absorbed all the heat from the block and exhaust. The more heat in a wire the more resistance to current flow. The solenoid can draw 30 or so amps on initial contact of the switch. That juice has to go from the battery, all the way to the switch and back to the starter. If the wire is hot the result can be no contact of the solenoid. If you install a remote starter relay you can reduce the draw to 2 amps through the start switch. You also have no live wires running anywhere to or from the starter unless the start switch is engaged, then it's only the cable itself.

Of course I'm not an expert, these are just my opinions and observations.