I put a plastic TR6 fan on my 4A many years ago, and just last year needed to replace the fan belt. Not sure if this is the same fan from TRF.
It's a tight fit, with only one inch (or less) of clearance between the fan and the radiator, but I have never had a any problems. That said Water temp is very consistent, even on hot days, and I have never overheated. I did have occasional overheating with the original metal fan, and this solved the problem.
If you remove the spark plugs when adjusting the valves there is no problem turning the engine with the fan, or by pulling on the fan belt.
One heads up.., in order to change the fan belt with the plastic fan installed you will likely need to loosen all the the radiator attachments, loosen the top hose and push the radiator towards the front of the car, in order to get the original belt to pass between the fan and radiator.
And remember, many of us 4A owners have found a need to loosen the engine mount bolts and lift the engine up a inch or so in order to get the belt between the fan extension and the cross support. I have found this is necessary when changing the belt with the plastic fan installed.
So make sure you put a new belt on as you won;t want to change it for a while
. I used a The Gates number is TR28386 (7/8 " x 39 1/4" - 22 x 995 ),which is available at NAPA under part number 25-2437 or 25-28386. There have been several threads on fan belts so you might want to check the archives before purchasing. I had a problem with the belt from Moss (squeaking), and needed to replace it with the Gates
Good luck