so your car only heats up only at idle, indicating that most of your cooling issue is likely going to be air flow related? odds are your fan set up has a lot to do with it. if your car has a mechanical fan on it without a shroud, adding a shroud essentially turns the fan into an air pump, by creating a housing for it. to make a water analogy, think of how much water you would move via spinning a little 4 inch diameter impeller on the end of a 1/4" shaft thats chalked up into the end of an electric drill, that's dipped into a bath tub full of water. take mental note that the volume of water is barely disturbed in the bathtub. its not hard to picture that in your mind. now imagine that same 4 inch impeller with a housing around it, to form a water pump and take note of how much more volume it moves. the volume of water being moved is multiplied multiple times over via surrounding the impeller with a housing to create vacuum.
for whatever reason, after 1958, the only factory cars that got fan shrouds were AC application. after 58, when the factory ran a shroud, they also ran a clutch fan and smaller pulleys most of these years. ideally its nice to run a clutch fan if there's space for it as they rob less power from the engine than a fixed rigid fan. the way the clutch fan operates is many are thermostat activated on top of friction/ oil activated. the thermal spring activates when it gets hotter so it doesn't create as much drag prior to operating temp, and its otherwise designed to slip at higher rpms associated with highway driving and hard accelerating, which coincides with conditions in which you're already getting constant feed of natural air from the road. if you run a clutch fan, you MUST run a shroud.
technically you don't even need a fan on the car at cruising speeds. also, in theory, having a fan shroud on it can slightly hurt cooling on certain applications at cruising speeds as it can actually slow/ block air by creating more turbulence/resistance at higher speeds. you will notice a lot of tight fitting electric fan set ups typically have plastic flaps to open up to help remove restriction at cruising speeds, and close at low speed cruise and idle. regardless of the fact, the pro's of having a shroud on your car, will far outweigh any cons you may experience, if any at all. ideally the shroud should cover the entire radiator core but on certain applications, including old factory applications, the factory would just run a loop around the fan itself with a bracket that mounted over the radiator. the advantage to that is there's essentially no high cruising speed restriction to natural air flow. the downside is your "air pump" isn't pulling air through the entire core. certain small cubic inch applications may work just fine with a shroud like that, however.
as far as setting up a mechanical fan on your car with a shroud; you do not want to place the fan as close to the core as possible, like you would normally when not running a shroud. if the fan is buried too far inside the shroud opening, then the fan will create turbulence instead of pull/vacuum. the fan tips should be halfway into the shroud and halfway out, if not, slightly further out, is better than being slightly too far in. fan tips should ideally be only around an inch or so clearance between it and the shroud opening as ive illustrated below. in other words, you want to stuff as big of a fan as you can. people talk a lot of trash about flex fans but on really tight clearance applications, those 7 bladed ones are bad ass and fit when nothing else will and they work amazing with a shroud. that one inch clearance gap rule can be impossible however on a lot of factory set ups. many of the stock clutch fans were larger than anything you can buy over the counter, and the way the engine sits in the car, the water pump flange isnt centered in the shroud opening, so many applications tend to be too tight on the bottom with a huge gap on top. in those scenarios, all you can do is average it out, short of redesigning the shroud.
a way to make a shroud for your custom application thats extremely easy, and can be done on a saturday morning, is simply build a sheet metal box with tabs on it that either bolt over the radiator mounting hardware that's already present or to the otherwise surrounding sheet metal. establish fan diameter and where the center of the opening needs to be on this sheet metal "box." use a sharpie and a string to draw out your opening to cut out with a body saw or cut off wheel and you're done. its not the most efficient shroud shape but it'll work great.
as far as electric fans, they have a time and place. with some applications theres just no other way. quite frankly, id recess the firewall on a custom car, long before i put myself in a position to where id have to run an electric fan because i didn't plan ahead. its no secret that i don't care for them. i think they're often the one thing on a classic car that looks out of place and not period correct. they look cheesy. most of them cant pull half the air as a properly set up mechanical fan and shroud, and they're noisy. the benifit to them however is they spin at high rpm when the engine isn't and they dont rob power from the engine. you will notice that all electric fans are shrouded units, even though the shroud doesnt always cover the whole core.
as far as electric pusher fans, i have found them to basically do nothing. you will notice there's probably not a single factory electric fan application that exists on a stock factory late model car. they block air at cruising speeds and they create tubulance via blowing air that bounces back and forth over and over again after being preheated by the radiator core, prior to actually feeding through it, basically blowing hotter than ambient temp air through it, at a slower speed.. an electric fan loses over 60% of its efficiency as a pusher instead of a puller.