Mr. Crenwelge notes the use of thicker cores in his post above. Sometime before 1982 my original core, which would have been for the original 228, was rotting and leaking. I took it to a radiator shop near where I worked, and the man told me the best fix would be to have a new core installed. When I picked it up he told me that he put a bigger core in. I did no measuring before or after the fix. However, since the 1980's my temperature gage goes to 180 and never moves. Like the original poster I drive a 270 in Virginia (VT CE 1987.)
I took a measurement today with a straight ruler inserted on the side of the core then pushed to the stop. Mine measures 1-1/2 inch from that stop to the visible front of the core. I don't know if this is any help, but if your measurement is less than 1-1/2 inch then that would confirm a thinner core.
1950 GMC 100 Panel Truck, 1958 270,
3-spd Saginaw with BW OD,
3.73 12 Bolt Differential from a 1966 C10,
Front Disc Brakes
Bought it in pieces 9-1976, driving since 6-1978
Follow in the DITY