The extra duration on the exhaust side goes with the extra lift; they are or where, back in the day, intimately tied together. This was a compensation for the poorer flowing exhaust system; from the valve to the rear bumper. It has been long known that the factory-LA SBM heads have a bit of a problem on the exhaust ports, and log manifolds just aggravate it, as do the teensey pipes and restrictive factory triple pass mufflers, and choke tailpipes. (exaggerated for effect).Magnum heads have better exhaust port shapes.
Without that deficiency, the split is no longer required.
However,that split can be used in another way. It allows you to move the cam ICL around a bit, to fine tune a combo, without destroying the overlap.This is useful on the track.
For a streeter it can be useful to drop cylinder pressure if encountering detonation; 4* can kill up to 5psi and maybe put your combo back into pumpgas. Obviously if you get the Scr right in the first place then, you wouldn't need the adjustment nor any longer, the split.
It can be used in other ways too, like thumpr cams.
Three inch full length exhaust, they say, is overkill for a SBM. I have dual 3s not for the power, but use the extra metal to extract heat from the exhaust, before it gets to the mufflers. In this way the exhaust becomes more dense and slows down on the way to the bumper, and the engine (it is my hope) will not have to push the exhaust out.I can't prove it, but I think it's working,lol.I'd like to try 5 or 6 inch ovals up front.... maybe I could get away with no mufflers at all,lol.
I ran the Mopar 292/292/108 cam for a season., at 11.3 Scr, in my streeter. I felt the lower rpms were soft so moved the ICL a few times without much improvement. At 100* it lost the zing at the top and picked up a bit on the bottom, but man was it hard on gas. I attributed the loss of Zing to the messed up overlap; I could be wrong. In any case I pulled it out ASAP, and sold it. After that I went down 22* and hit paydirt with a 270/276/110. That 6* split allowed me to some room to experiment with the ICA, and not mess up the overlap too bad.When you don't have much(53*), you can't afford to lose effectiveness. That was a sweetheart of a cam.