Before you start throwing money at it you should figure out what’s wrong. There are 26” A-body fan shrouds, the 73+ A’s did have a 26” radiator as an option. A 318 should have no issues with overheating with a factory 26” radiator if everything is working as it should. You could have any number of issues, faulty or missing thermostat, mismatched pulleys, a 6 blade water pump with pulleys for an 8 blade or vice-versa, incorrect timing or fuel mixture, etc, etc, etc.
Switching to an electric fan and doing it right means wiring in fan relays, some kind of controller for the fan, and being able to pull enough amps to power a fan large enough to cool the car. That’s not a small thing, a stand alone electric fan needs to pull at least 2,500-3,000 CFM to keep the car cool. That means a starting amp draw of close to 30 amps or more. Done incorrectly that's more than enough to fry the stock wiring harness, and some of the alternators for these cars were only 46 amps. No more than 60. Half that to the fan doesn't leave much for other stuff.
And those radiator and fan kits are almost always bad news. That flat fan shroud is a bad design, it's just the cheapest way to do it. The fans specs aren’t published. It doesn’t even say if it’s a one or two speed fan, how many CFM it pulls, how many amps. It’s just a cheap add on from a company that builds radiators, not electric fans. Most of the electric 16" aftermarket fans DO NOT pull enough cfm to be a stand alone replacement.
I run electric fans on my car, I'm not saying that's a bad idea by itself. But most people fail in running electric fans because they just toss some old fan on there, don't wire it or control it well and are then surprised when it doesn't work. If you have the 26" radiator and it doesn't leak, you can get a shroud for it. If it needs to be rodded out that's a lot cheaper than a new radiator and the conversion to an electric fan. And there's no reason a mechanical fan and the factory radiator shouldn't be able to cool a 318.
I run dual speed, dual electric fans from a Ford Contour V6 on a 26" champion radiator to cool my 400+ hp 340. It works great, and cost less than the ColdCase radiator by itself. Not saying that's what you need, but you can see what it takes to do an electric conversion correctly at least, it's not just a "bolt on" deal. Relays, fan controller, 100 amp alternator, amp gauge bypass to keep from frying the bulkhead connector, lots of heavy duty wiring, etc... My "new" '74 Duster- or why I need a project like a hole in the head