Geyser on my radiator

Hi all ,

well i rebuild a small block 318 with some new and used parts .
yesterday i doing the first start and at the beginning it's ok , the starter is ok and the idle is exellent but after five/eight minutes i have the colling fluide jump as a geyser on the radiator .

- First the radiator is not oem , its radiator of Mazda 323 with reducing hose for the adaptation and it's fixed under the level of a oem radiator ( the caps is at 3/3.5in under the level of the stock radiator ) .

- Second the geyser wen the engine is hot is important but at the exhaust i have not some smoke white .


one idea please ?

excuse my bad accent i am not américan , my respect Gi ....