*3.42 or 3.73 (not more horses, just quicker - provided you ALSO include a limited slip differential)
2.50" full-cat-back exhaust system
1.6 roller rockers with upgraded valvesprings
cold air intake
*reprogramming to maximize full advantage of the above
*best bang-for-the-buck, usually improve the 4L60E's life expectancy when done properly
Here's a list of basic recommendations for making a b-body faster. The important thing to remember is that our cars are old and somewhat unique compared to the usual Camaro and Vette. Our cars are heavy and have an LT1. With that in mind, talk to the guys here on the forum or a local B-body...
A great thread if you're in the mood for a long read and ready to give it due consideration.
My previous list is how every Impala SS (and 9C1-LT1) should have been, standard out of the showroom.
Think of them as GM's omissions; they should have been de rigeur. In fact, they still should be.
More 'serious' 'intense' mods require more in-depth research, not just because they become more personal.
I don't necessarily disagree with heads&cam improvements, but not everyone can agree on which heads&cam is 'best'; this is where personal taste becomes a primary deciding factor.
It's easy to agree that:
3.42 or 3.73 are far better than 3.23 or less - unless 500 miles per tank is not enough AND ALSO most important over all else
if you do not install a limited slip differential with 3.42 or 3.73, it's either a mistake, or it's for comedy
the OEM exhaust is too restrictive, and $500 installed releases at least 15 horses and sounds bitchen
1.6 Roller Rockers and better valvesprings improves and widens the IronHead's LT1 powerband
the intake should get 'swiss cheesed', at least
the OE calibration is for unrealistic drivers driving for unrealistic anachronistic tests, not for enthusiastic driving, and even those who want over 500 miles per tank still deserve better potential than the OE calibration
if every Impala SS had come equipped off the showroom floor as above, Caprices RoadMasters and non-V4P Fleetwoods would have been more popular, and maybe even better as well - why not V4P Caprices & RoadMasters?
(5.0L V8 'base engine 'Caprices would have been better than 4.3L)
I would buy a higher stall converter before paying for roller rockers. A good converter will improve 60' and 1-2 shift recovery. I don't consider rockers to be a 'low hanging fruit' modification (can't easily see the difference on time slips - YMMV) .
The factory iron heads are more receptive to heavy porting and bowl work than most aluminums, and accept the larger valves without danger of beating up the seats. Raising the intake ports (and matching the manifold runners to that) improves the charge. That requires more work than simply pulling out the plastic and ordering a pair of off-the-shelf heads, but it's a labor of love.
The Comp 07-305-8 (276-HR) is good for moderate reprogramming, and still provides good idle. I feel it would still run acceptably with even the stock PCM flash, but that scheme leaves a lot on the table.
Good springs and titanium 7° retainers top off the job, and it nets probably a bit more than 15HP above the stock rating. Shifting 1-2 at about 50MPH (5,000 + RPM) will crack the sticky Nittos away from the pavement for a prominent chirp, top end is good, and loss of torque just above idle is negligible. For s street car, it's been pretty satisfying. It can still do 26 MPG on a trip below 80MPH, so little is lost there as well.
Obviously, there are a hundred ways to skin a cat, but this retained stock-ish mannerisms with a dark side in reserve.