What year is the 261? If it's the newer type with the offset in the oil pan rail beside the pump, there will be a bit more room for a bigger pump than the 54/early 55 engine. Either way, you'll probably need to do a little surgery on the oil pan sheet metal, as the GMC pump is MUCH larger than even the bigger oil pump used on the later model Chevy engines. Have you considered the added load on the distributor drive gear the extra volume pump will create?
I think I'd choose to use an external, belt-driven oil pump like the dry sump oil systems on NASCAR engines use. They're driven off the crankshaft snout by a toothed belt. There is also a way to put the taller, high volume gears from a Chevy V8 oil pump into a stovebolt pump housing by adding a spacer plate between the pump body and the base plate.
Jerry
