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If your cost constrained but already have a choice; the Al. headed LT-1 is the way to go, assuming we're talking a 90's era reverse coolant LT-1. (Not L-98s! if it's L-98s go with the Vortec if you can't afford anything else.) This engine is ready to go with a lot of cam, valve train, compression, and stronger than typical internals. You can add more cam and other race quality parts without having to machine the daylights out of the heads to get there. Then the Al. heads are 50 pounds lighter which will count for a lot in a short wheelbase car like the Vega when it comes to getting weight transfer to the rear.
There are some folks that say you can get more power out of cast iron heads because iron's slower rate of heat transfer allows more heat for pushing on the piston. This, however, is not necessarily true; the faster heat rejection of aluminum allows more compression. In a race between higher compression versus slower thermal conductivity, it appears that more compression has a slight edge more often than not. But it's in the noise, some dyno drivers get more power out of cast iron others aluminum, but nobody gets one or the other consistantly. So this tells you there are other more significant factors at work. But your not running on a dyno and you need to take in the whole car as a system. As such weight transfer will be a big issue. Basically the more you can put on the read wheeels and the less you have to rotate off the front the better will be the tire adhesion and the faster the car will be.
Since you don't have to spend money doing major mods to the Vortec heads so you can run a decent lift cam, springs, screw in studs, etc. you can put that cash into a lightweight crank & rods, an aluminum drive shaft to help get the engine to rev faster. While your at it, dump the slush box and get a 5 or 6 speed in there with a lightweight flywheel which will really punch up the motion between the lights if you can get it all connected to the ground.
Speaking of getting power on the ground, if you stuck with living with the automagic remember that as the stall speed goes up the efficiency of the hydraulic coupling goes down. So your in a race to find that combination of stall speed and engine power that nets the minumum ET through the lights. However, this has to be looked at within the constraints of the chassis being able to put that power on the ground. In-other words you'll need to expirement here. If your loose coming out of the hole higher stall will probably only worsen the situation. If your bogging out the hole more stall can help. But you need to keep in mind that chassis set up and total gearing (gear ratios, tire size, traction) all play a role here.
Since this car is fairly light and geared pretty stiff, you can run a pretty rasty cam and a high stall converter the cam manufacturer can and will advise you (so give a few a call) and take advantage of top end horsepower rather than bottom end torque that a heavier car would need to launch. A solid roller is the way to go. With either head you've got to do away with self aligning rockers and put in pushrod giudes.
Problem you seem to have is financial, it costs money to be fast. The more competitive you want to be the more expensive it becomes. And this thing becomes self feeding. The more you make it fast, the more expensive the parts become, the greater the risk of blowing it up becomes, so you have to put in even more expensive parts not necessiarily for power but to just keep points A and B connected to each other.
When I was a kid the speed shop in my little SoCal home town had a banner behind the counter, it read "Speed Costs Money! How Fast Can you Afford to Go?". This old axiom is as valid today as it was 50 years ago.
Bogie
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