The Carb spacer would help out if you could spare the hight, but it sounds like you can't unless you cut the hood open and put a working or fake scoop to add more room.
Also there are 2 main types of spacers: 1 Open- for more top end, because it increases the plenum volume. 2 ones that have 4 holes- for more low end power, because it increases the velocity of the air fuel mix.
I wouln't put one of the butterfly air cleaner jobs on unless you are going for looks- the only thing it would do it get colder air for your car. You probably would not notice any difrence.
One other option is a cheeper Nitrous system. one that goes from 50-100 or 150hp you can find one through summit for $350 from edelbrock or $370 by NOS.
I have a setup almost exactly like your 350chev 4bolt, preformer intake, Mild Cam, headers, dual 2.5" exhaust, HEI, with a Th350 behind, and 12bolt rear.
In my opinion more lower end power to really get you moving fast from the start, with out having to run the 4.11 gears is the best way to go.
If it was my choice,
I'd get a bit higher stall speed torque converter(just allows engine to rev higher before it takes off [on mine the power dosen't hit till about 2000-2200rpm $80-100)-
A K&N filter(if you don't have one)(10-15hp for about $50)
A small Nitrous system(50-150hp for $350-400)don't go too crasy on a 2bolt and if it just has cast pistons.
I've also heard that roller rockers could help some, but I'm not sure how much(10-20hp?? for $150)
Also your 600 could be limiting the hp just slightly a 650-700cfm would be better sooted in my opinion
Hope my research for my engine can help you out some.
