It's not uncommon for rubbish that is in the cylinder block to be dislodged when an engine is rebuilt, & this of course finds its way into the radiator, partially blocking it & causing heating problems.

I suggest that you drain the radiator enough so that you can clearly see the top of the core. You will be able to see only a small number of tubes, but look to see if they are blocked with rusty looking particles. If they are even partly blocked, it may pay to have the radiator reverse flushed.

Hopefully this will fix the problem, but if not, then a full tanks-off radiator service may be called for.