Shane,
I'm not familiar with a 261 as per say, but I'll see if I can shed some light on the situation. The older model engines had poured babbet rods and mains. Somewhere around '53 or '54 they GM changed to replaceable inserts. If you have inserts, they (along with the crankshaft) should be machined within tolerance and shims won't be needed. You could get some plasti-gauge and check the clearance (should be about .0015 to .003--check your manual) It's been 30 years since I have rebuilt that type of engine. Maybe someone else can shed more candlepower to this.
Truck Tinker
