In the meantime (since my original post), I've had my 455 marine engine built buy a competent engine builder who knows his way around a 455 Olds (and Buicks and Hemi's etc...) and I couldn't be happier.
No oil restrictors in the mains, only restricted pushrods and good old fashioned attention to detail.
I went for coated bearings as insurance and am using outside grooved cam bearings with aftermarket rods (Olds always seem to spin the rod bearings first because the factory rods are crap for HD marine use).
With a 10 qt oil pan and HV pump, I never pump the sump dry and all he did was some standard clean-up for the oil returns.
It's a solid 400-425HP motor and I can run it all day at 4500 without worry (with an 'A' impeller, it tops out around 5000).