Serial numbers "Gon4 j307". what do I have?

[Kurby]

Hey all,
I purchased a 1959 grumman van that has been sitting since 1980. The guy I bought it from said he had a new engine installed about 1980. Its a straight six with the code " Gon4 j307" on the side by the starter.

What size is this engine? what year??

Thanks!