here is a link for some common fan dimensions for thickness. you could always put the fan on the front side of the rad if required, but if you planned on the backside (engine side) then just ensure you have room plus some safety dimension, the air needs to go somewhere. it seems like the thinner fans are made more for use as a helper fan, not a stand alone engine cooling fan. if you will have a/c you will need all the cooling you can get, likely. if you look at the rad and fan that came with the engine's originally intended purpose you could get an idea of what was intended to be used for cooling. you could use 2 fans so the motor part wouldn't be inline with the water pump snout.
https://becool.com/pages/fans
and another site with electric fans
https://www.jaycorptech.com/c-14-spa...yAAEgKyavD_BwE