LMC catalog? I do not see 216/235/261 Chevrolet or 228/248/270/302 GMC thermostat housings (it may be elsewhere in the catalog)?

Even if LMC had a 235/261 thermostat housing (which should work with a GMC engine?), the GMC housing has different hose ports (which is useful for those of us who have water-heated intake manifold). You might want to ask on the oldGMCCtrucks.com discussion board.

Tim