General tuning theory due to exhaust change

[Bailey28]

84 El Camino.  Stock 350 Chevy crate engine. 

Went from a single 2 1/2" mandrel bent Y pipe back system with a single Magnaflow 14" straight through muffler to a 90's Z28 style muffler which is now transverse mounted behind the axle. 

I noticed the tune feels not as crisp now.  Both mufflers are 2 1/2" inlet however the Z28 muffler is a baffled/ louvered tube oem style instead of straight through. 

I have a vacuum gauge in the car that I monitor while driving.  Idle vacuum was 1/2" better with the Z28 muffler and was 1" better at very light cruise. 

Heavier throttle input results in the vacuum gauge dropping a few inches lower than with the magnaflow muffler. 

The magnaflow was too loud for me.  The Z28 muffler sounds perfect. 

Any ideas on whether to start tuning richer or leaner with this new exhaust? 

[qjetsrule]

Is it a (CCC) computer controlled carb? The last test on this chart is for restricted exhaust.

[Bailey28]

No CCC, the carb I have is a 17080206.  Stock jetting in it, 72/51M jet rod DP secondary rods.  It is dialed in and runs very well. Awesome fuel mileage.

According to the vacuum gauge chart the gauge responds as in the first scenario. 

I'm thinking one of two cases could be possible here:

First, the Z28 muffler is more restrictive thus leaving more residual exhaust in the combustion chamber thus requiring more fuel to offset this condition.

Second, the Z28 muffler could be less restrictive and offers better scavenging of burnt gasses thus requiring less fuel. 

[Bailey28]

UPDATE:

I switched PCV valves to a higher flowing valve leaning the mixture. 

I found that it wants less fuel with the new muffler.  I got my crisp response back. Feels like EFI again.