hi recently buy a chevy van camper

i need to do a head gasket job but i dont know wath is the engine on it
is not writhe on my register like my others cars I thinks is because is to old !!! so is writhen on the side of the front wings "CHEVY VAN 20 "

I think is a g20 350 5.7l but could somebody can confirm me please
i can put picture if you need

and where you suggest to buy a part ???? i found partstraint.com is it good

thanks a lot gang

Max