Newer aluminum intakes are better designed for the flow of the air/fuel mixture to the cylinders, meaning maybe a couple of HP. Aluminum dissipates heat faster. If you get an aluminum intake with an air gap between the valley pan and runners it can reduce air temperatures, meaning you gain a couple of HP. If you get one that matches the carb you plan to use you won't have to mess around with adapter plates which increase the chance of an air leak and can also create unwanted turbulence in your air stream. You don't have to worry about rust. And you save a couple pounds of weight!
Aluminum in general is an improvement on a lot of bolt on parts used for an engine. From water pumps, intakes, heads, valve covers and more. I've also heard of people claiming aluminum oil pans help keep the oil cooler, but for me unless the engine uses a factory aluminum oil pan I'm not going to swap as aluminum's biggest fault it is doesn't dent it cracks and shatters. It wouldn't take long after a stone or something shattered your oil pan before all your oil is on the road and your engine is grinding metal to metal.
Thanks. The car is giving me trouble again and I suspect itís the carb. I talked to an Edelbrock technician about what I would need to make a 1406 work and he was saying it may be best to go with one of their intakes because of heat issues. I took what he said with a grain of salt because I know itís their job to sell stuff, especially expensive things. Will I be fine using my original intake with a 1406 and spacer. Anyone else have any input?
A spacer negates the heat cross over function for smooth idling when the out side air is cold (not needed in SoCal or Texas or the Deep south because we'uns can't even splel kold).
As Deadwolf pointed out a newer aftermarket intake will add power due to better fuel distribution (no right hand turns in the runners like on a stock manifold made to keep labor costs down) and better flow due to larger runner volume. That said it doesn't have to be brand new and shinny. There are no moving parts to wear out on an intake manifold. The only thing that can happen to them is they can warp (a quick trip to the machine shop fixes this) or have broken or cracked threaded holes that require welding up, drilling, and then tapping a new threaded hole (once again a machine shop can fix it).
I used to make good money buying used Holley carbs at swap meets and rebuilding for sale to my customers (I would have maybe $50 bucks in the carb and rebuild kit and sell it for over $100. The same place sells intakes in all shapes and brands (Edelbrock, Weiand, Holley, etc.) in various condition. If it is obviously broken then I wouldn't pay more than scrap value for it (especially if you are buying on-line with shipping added on.
The kick down rod is the analog input that resembles a throttle sensor position sensor on a computer controlled car. That rod in conjunction with engine manifold vacuum allows your transmission to drop into "passing gear" (next lower gear from where you are now, up until the governor kicks you back into the higher gear again to prevent over reving the motor). The kick down linkage isn't a carburetor problem. That is every carb will require you having to figure out how to attach it properly as it is part of the transmission not the intake manifold or the carburetor.
That is the only effect it has on the car's driving. Without that rod (that is reproduced for a Rochester carburetor) you won't have a "passing gear"; but you can still down shift manually if you want to pass. . The Rochester rod can be modified or adjusted to work with a Carter carb; or in the cars I used to work on, a Holley carb. If you can not do it your self there are grey haired old mechanics that still remember what a carb is; and can do it for you (for a nominal fee). It isn't rocket science, and with the kelp of a Chiltons or a Motors service manual you can do it your self.