I’ve put a big block in my 1970 with a mechanically fan and fan clutch. When I put the car together, it was originally a 350 car with ac, but I bought it without an engine and most other things. I bought a radiator years ago that was in stock at the local auto parts place when I first put it on the road. I didn’t realize it at the time, but it seems to be the thinner radiator (maybe 2 row vs 3 row?). I never installed the fan shroud at the time because the fan sits about 1/2” off center from the shroud. So the top of the fan is really close to the shroud and the bottom of the fan has more space than it needs. I’m worried about it rubbing. since the top radiator bracket is a repo, I wasn’t sure if it was made right and was about to drill it for higher mounting bolts so the fan is centered. That’s when I realized that the big block shroud when installed properly sits about 1/2” or so away from the radiator. I guess the difference in thickness between the standard vs heavy duty radiator. (2 row vs 3 row?). Even without the shroud, it never gets hot on the highway, even in 100 deg F summer. And now that I have the base timing set right, it doesn’t overheat when idling, unless the hood is open. Which is why I want to install the shroud. However I’m concerned that if the big block shroud is that far from the radiator, it won’t work as good. Therefore I’d probably have to figure out how to get the gaps filled. If I have to modify a bunch of stuff, I might as well look at some other options. I didn’t want to run electric fans (although might still consider it - I was looking at radiators4less Dual fan Setup made for this car, at $140, it wasn’t too bad), but I’m concerned about durability and if it breaks on the road, where do I buy parts? Although I guess it’s getting harder and harder to find fan clutches and stuff on the shelf too! I was thinking about maybe trying a small block shroud? Or trying a 3rd gen shroud, but don’t know if the holes will line up. Gotta measure one on a friends car. I didn’t want to buy a new radiator, but that may be an option as well. Suggestions?