Note that 65mm converts to 2.559" or near to 2 9/16" (2.5625"), though this length is often referred to as a nominal 2½".
As Larry stated the Chopper Lump is generally regarded as "The Best" though there are pros & cons to this. Chopper Lump requires the bbls & lumps be made of the same mat'l. As the lump in the dovetail construction is a seperate piece it can thus be made of a higher grade of steel than would normally be used for the bbls. As the breeches of both types are joined by brazing any previous heat treatment would be lost.
Dovetail Chopper lump & the mono-block were both developed for the purpose of eliminating the necessity of brazing so tube strength could be increased by some heat-treatment.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra