now thats a blast from the past .... I had a 6x2 intake on a Buick nail head at one time I got in a trade.
at one time those intakes were dirt cheap at flea market swap meets, but with the rat/rod craze the prices have increased a great deal.
a great deal depends on its condition and both the engine they were designed for and the condition of the carbs if those are in the package, but if its in EXCELLENT CONDITION WITH OPERATIONAL CARBS IN EXCELLENT CONDITION, $3000 PLUS is doable, for the time period correct hot rodder crowd.
(but highly over priced for how they run in my opinion)
take the effort to have the carbs rebuilt, the intake cleaned and the linkage installed and adjusted correctly, looks and function are VERY important
heres a couple that are NOT in excellent condition
OFFANHAUSER, and WEIAND SOLD SIMILAR INTAKES
NONE OF THEM PRODUCED REALLY GOOD HP BUT AT THE TIME THEY WERE COMMON THAT WAS HARD TO DETERMINE AS DYNOS WERE RARE, OFFANHAUSER BUILT SEVERAL RATHER UNIQUE INTAKE DESIGNS
with every design change you get a compromise in some area, theres no free lunch, but there are choices to be made and at times those compromises benefit the application,
offy dual port intake manifolds, use a smaller separate lower set of intake runner passages,
to radically boost intake manifold runner air speeds too in theory increase the fuel to air mix and port filling efficiency
this actually works rather well in the lower rpms and as the secondary larger upper tandem ports open up that faster lower current of fast moving air tends to help the transition or slower air mix and keep up,the problem is that the divider wall between the ports is thick enough to limit the total available cross sectional area so that while low and mid rpm torque tends to benefit from the designs characteristic flow rates the peak power is slightly restricted over what you could potentially get from a more common, single larger, cross sectional area available in the intake runner port manifold like the wieand or edelbrock designs
RELATED INFO
viewtopic.php?f=55&t=4302
http://www.vapinc.com/carbandkitsspeed/main.html
http://www.vintagespeed.com/