What 78 said. There are basically two styles of connectors used over the years at the distributor end (with minor cosmetic or ancillary differences of course) and you now need the "newer" style.
I consider it the better design, as it's less prone to corrosion from moisture getting into the old recessed pocket, and less likely to pull the end off when you're just pulling plug wires for whatever reason.
Most electronic ignition systems when over to that style many years ago, but with some (including MSD) having distributor caps with both old and new styles available. Probably just to cover people's personal preferences I'm guessing. Otherwise no real reason to keep the old style unless there's a clearance issue for some reason.
So even though you may be able to get a replacement cap for your distributor to match your old wires, I would get the new wires to match the new cap instead.
Besides, adding more spark energy usually means old and tired wires get taxed beyond their limits.
If your existing wires are fairly new, I can understand why it would be nice to use them. Wires are more expensive than caps. But I still say it's better to use the newer ones.
Maybe you can have your cake and eat it too though? If your wires are new and you can't return or resell them, you might be able to get the correct cap for them that fits your distributor, then when it's time to replace the wires in the future you can get the modern set and re-install the cap with the spark plug ends on it.
Just a thought anyway. Something to cloudy up the subject and give you more things to think about and decisions to make!%)
Paul