A 261 uses the same crank as a 235. I've built several 235s, and never had to sand a thrust bearing. Sanding thrust bearings was a procedure done 40 years ago to fit the parts. The shoulders on the thrust bearing take the fore and aft movement of the crankshaft. Typical values are .003" to .009" thrust clearance on the thrust bearing, taken between shoulder of bearing and crank thrust face. The insert should just slide on. The Clevite or Federal Mougal bearings should fit correctly. A machine shop is needed to insure that the bearing is the correct one, and the crankshaft has been machined correctly.
I once had a 235 engine come apart due to a failed thrust bearing. The crank walked back and forth and beat the hell out of the rods and chewed up the bearings. The thrust face on my store-bought crankshaft was poorly machined, and it ground down the thrust bearing inserts in about 10K miles.
good luck
chip
