In the 67' & 68' era, I had just started rebuilding (not building) engines, I was still mainly doing the heads.
I remember when the 4 bolt main block was introduced, it was needed badly as the 2 bolt main block was prone to cracking through the main webs. Also in that block chevy addressed a problem of cracking around the cylinders, through to the water jackets.
The earlier chevy rods were too weak and required a lot of massaging to last a race or two, the later (2.1 journal) rods were stronger. TRACO (when possible) used Carillo rods and Bartz used the 427 rods, both worked well.
I don't know of (or remember) a HP gain or loss related to the journal size, it stands to reason that friction and reciprocating weight would have an affect, but remember that TRACO minimized the reciprocating weight every way possible and used Hard Chrome plating on the bearing journals, which has about a 20% better slip coefficient.
As I remember it, TRACO always used the 4 bolt main block because it would take a lot more punishment, it's the old "You must finish to win" philosophy.
"In general, were you in search of these types of smaller horsepower gains?" Yes, but not if it significantly reduced the engine's reliability.
An engine's ability to "Breath" is a huge factor for HP, which is why modern engines have multi-valves per cylinder, requiring overhead cams. Large bore, short stroke, 4 or 5 valves per cylinder, direct port FI, individual cylinder timing and fuel control, allow some small 4 cylinder, normally aspirated engines to put out gobs of HP.