Radiator/reservoir tank temperature differential question

[Rederic]

Radiator coolant reservoir temperature differential question
1991 LT 31 Florida
1G motor

Good day!
Not sure if this is the correct place to post -
Recently had complete top and bottom end rebuild including everything new head etc.
Had the radiator tested to make sure everything was good.
Recently blew a head gasket after 40,000 km.
Just got everything taken apart machined reassembled.
The mechanic is very meticulous and took some measurements of the radiator compared to the reservoir tank.
He said that there’s already while idling a The 20°C differential between the reservoir tank and the radiator.
He said that there’s not supposed to be more than 10° differential and this indicates that my radiator is clogged up and inefficient.
He suspects this is the cause of the recent head gasket failure.
Can anyone confirm this differential specifications between radiator temperature and reservoir tank temperature?