I like to go with the size that Ford suggests from the factory..I have a '70 with a 302..I am running a stock type radiator, and a fan shroud..it runs at a normal temp, I would think a 3 row should be more then enough for a 351W
unless you are running A/C..I don't see why it won't work..why not give it a try since you have it already..if it begins to run a bit hot you can always switch over to a 4..just make sure hoses and t-stat and water pump are in good working order..
I have the standard 3 row 20 inch radiator in the 69 Mach. Previous ower used JB Weld to "heal" the overworked radiator. I'm told this was standard in 1969, for a 351W with no AC. I'm thinking a 3 or 4 row 24 inch radiator is in the near future, just for added insurance. Since you don't have the option to go wider, you might consider a 4 row. I guess it also depends on how you motor is setup.
Having been a frustrated overheated mustang owner for the past 25+ years I will tell you to not waste your time on an old 3 row. Go get a 4 row and you will be much happier. I got mine from Sacramento Mustang. They have them specially made for them. It gets very hot in Sacramento and based on others recommendations that is what I did. I couldn't be happier. These last two weeks I have almost stopped looking at the gauge. Plus, I no longer hear all the gurgling that I heard before. Its very nice to have a cool running car! The radiator was $199 and easy to install.
Don't mean to totally confuse you, but my vote is for a 3-row. When I was considering which radiator to buy, the 4 rows all looked very "aftermarket" (which may be what you want...).
On the other hand, the 3 row and shroud I got from Sacramento Mustang, looks almost identical to the 2 row I pulled out of the car (when it had a bone stock 69 351 2v in it). Fact is, it was running just fine with a ratty old 2 row in it.
After consulting an "expert" in my car club, the 3 row got the nod so that I can maintain the "stealth" appearance of my car.
The expert I consulted is a fellow who owned a radiator shop for 25 years and installed (and even built) many custom radiator set ups. Without having started and driven the car since a rebuild and mild hop up on the engine, I cannot absolutely say that a 3 row will work. But...my expert tells me I should have no problems.