I bought the cheesecloth radiator filter from a radiator shop & it came with printed instructions & servicing recomendations.

When fitting a replacement engine I have found that if the engine has been in storage for a little while, the crud in the block dries out & shrinks, causing it to crack into smaller chunks. When the engine is installed & used, the expansion & contraction caused by heating & cooling cycles tend to allow bits to break off & the next time you give it a good rev out when hot, another chunk, big or small, is carried up into the radiator.

My personal suggestion.
Remove all welch plugs from the engine block before you install it & clean out as much crud as possible with a long thin screwdriver, & wash out with hose. Remove block drain plug too if fitted & make sure the hole is clean as this will be usefull later on.
Replace welch plugs with brass ones, particularly the one behind the flywheel
Fill the cooling system with water only at first. Install filter & drive. Just keep an eye on the colour of the water. The rustier the colour, the sooner it should be checked, but leave it untill the next service if you like.

If there is a lot of crud in the filter, then refill with water & check again fairly soon. When there is very little or none, then drain & fill with COOLANT & recheck whenever you like. Make sure that you drain the block if you have a block drain plug whenever servicing the filter during the 'water only' phase. My radiator also has a drain plug, so I drain it too. Now that I have coolant in mine, I drain from the radiator fitting just enough to empty the top tank when checking, & pour it back in when i'm done.

When the filter catches very little or none, you can leave it out all together if you want, but mine is still in there.

The filter that I bought is a moderately coarse weave & anything that passes through it will also be small enough to pass through the tubes. It is also long, so a fair bit of it is actually inside the top tank. As an experiment, I completly blocked off the airflow through the radiator with some cardboard. Screwing the idle up to about 1200 rpm allowed the fan to suck it on & it stayed there.
I waited for it to get hot enough for the thermostat to open fully & watched the rate of flow in the top tank.
The filters restriction was so low as to be imperceptible & the coolant rushed by under the filler neck at great speed. Naturally if you suddenly notice an increase in temp, then a check of the filter is in order, but just remember, every piece of crud in the filter is another piece that didn't make it into the radiator to do its nasty work.

My 1200 has a new, but stock core & I drove it all through the heat of summer & the temp stayed right where it was supposed to be. The only thing that got hot was me.