jay d - from my research into the GMC Parts Books I have, it appears the front and rear block coolant core plugs for your 270 should be 2" diameter concave disc style (no lip). Dorman 550-028 matches the description, dimensions and style of the core plug for your engine. The original GMC Part Number is listed as 103898 - Plug, cylinder block (2" expansion).
The 270 camshaft rear bearing plug is listed as GMC Part Number 838127. That same part number is listed in Chevrolet Parts Books as Plug, camshaft bearing hole (cup type) with a 2" diameter. Chev's of the 40's offers their
Part# 838127 as the rear camshaft bearing plug with a diameter of 1-31/32" as a reproduction for the original GM part number.
Hope this may be of some help.
