My guess would be that the 6.1 and 6.4 in SRM (short runner mode) is very close to the same. The 6.4 in LRM (long runner mode) probably makes a fair amount more torque than the 6.1 down low. The car 5.7 intake without the SRV probably makes close to the same low end torque as the 6.4 in LRM, but gives up top end power to both the 6.1 and 6.4 in SRM.
I kind of think of it like this:
5.7 car intake = Performer dual plane (good low end, not as good top end)
6.1 intake = single plane (good top end, not as good low end)
6.4 intake w/ SRV = Performer RPM AG (best of both worlds)
Engine Masters did a test on a truck 5.7 which has SRV and it made a pretty good difference making use of SRV compared to locking it in one mode or the other. Which is why I really wanted to be able to use the full SRV capable 6.4 intake.
But in a lighter A-Body, not really sure more bottom end is what it needs so I would guess a 6.1 intake is more than good enough. I wouldn't hesitate to run one.
If I was looking at an aftermarket one, I would look at a high ram from Holley. Or a mid ram if the high ram was too tall. But no idea how they compare to the stock intakes.