I think most of the responses have covered the concerns that might be created in an effort to lighten a crankshaft. But if only as a recap: do not just chuck a crank in a lathe and start reducing the counter-weight dimensions, particularly concentric with the crankshaft center-line as even if material removal is required for piston skirt/pin-boss (stroke) clearance with most American V8's it needs to executed in an offset cam-ground fashion to preserve material sums on the counter-weights or you'll be in trouble with the balancing effort. I think that the statement made by Scat, as quoted previously might be based on their origins, that with the VW's flat fours as with these (180° opposing layout) and including vertical in-line fours (Pinto 2.0's & 2.3's these with 180° opposing crank throws for example) it has been popular to cut-down the counter-weights solely to "lighten" the mass.
It's not likely that even the poor machining execution of excessive hole drillings in the counter-weights would lead to cracks in the critical areas of the crankshaft that which leads to failures. But I have witnessed cracking happening in cast-iron cranks (rarely) about the machining and migrating in an unpleasant manor that didn't instill a lot of confidence in the unit; but with steel forgings (even those of "off-shore" linage) it would be very doubtful that this would really be an issue.
But in extreme applications of R.P.M.'s , loading and high frequency vibration being imparted to the crankshaft, particularly if the application is known to present greater than appreciated flexibility, it is popular to remove any concerns for the interruptions in even these surfaces presenting windage influences in the crankcase volume (a properly sized (smaller) counter-weight or an oversized one with big holes drilled in it?
), and any tooling marks that as unlikely as it may be, may create an issue, and here is where one can justify the reasoning for the "no-hole" balancing efforts (and they "wash-up" easier too!
). And all of this is to executed by the one doing the balancing effort! And if this person is capable, this also permits one to better capitalize on the fact that counter-weight values carried nearer to where it really needs to be vs. sums in excess somewhere else attempting the same effect, though the influence is from afar, will prove to be a lesser sum. So although a one hole correction works for the entire mass carried, it's established singular positioning is an averaging of the hole, so is this specifically really where the excess solely is, or just the identified fix? And, it would not be recommended that someone "prepare" a crankshaft with any "lightening" effort, without being aware of the requirements in the balancing first; otherwise as stated previously, you might create quite the challenge for the balancing process, which generally would equate to an enhanced billing sum from the shop for their effort to rectify.
Probably the greatest concern is whether the crankshaft as originally created, was this executed properly? As often the manufacturer does not grasp the effect in of the common practice in the use of a singular crankshaft forging with the placement of the counter-weight sums being constant (and hopefully correct for something?
)and the various crank-pin and bob-weight mass values and placements due to stroke variances. Often this proves to cause the requirement of excessive additions of metal which is being required due not to the fact that there is an insufficient mass sum, it's just not in the right place; so now your crankshaft picks-up weight, to offset the manufacturing errors!
And just to clarify: as far as your steel crankshaft weighing significantly more than the cast iron unit, this has more to do with the sums of material present in the comparison, not the fact of the material differences. I don't know the exact consistencies of the two being compared but some cast irons can be lighter, or can be at least as heavy or more than as some steels, I not sure here if difference would really be significant but......?
As for the 391 steel cranks, my experience in the past is that they come up short on the counter-weight sums and more often require greater heavy-metal additions; this as compared to the O.E.M. 427 cranks.
Overall in something like an FE engine don't get all worked up over the fancified possibilities, as this unit in all but it's rarest forms wouldn't truly benefit from the efforts, and yes "all the details & little things do add up", but just follow sound and proper processes and spend you time and money wisely.
Scott.
