OK, here is a picture of three rod bolts. One is from a Mopar Performance set for a 340 engine. One is an ARP rod bolt for a 340-360 engine. And, finally, one was removed from a 1983 slant six 225 (cast crank).
There is practically NO Difference between the various bolts. All weigh the same, about 42 grams with nuts.
The Mopar Performance bolt is exactly the same as the slant six one but the nut is slightly taller than the slant six nut.
The ARP bolt head is shaped slightly different, head is slightly narrower but also slightly thicker. The ARP nut is as tall as the Mopar Performance nut but it has a flange built in.
The shank part of each bolt has no measurable differences.
Not exactly a scientific comparison, but it's close enough for me.

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-MattMan
LEANED & MEAN