6 cyl a230 ?s

mid 12s with a 318LA and A230 is doable, but not on a budget, and it won't be very streetable.
In a 3400# Duster(car and Driver), yur looking at 340 hp, 4.10s and 27 tall tires. Doable with a 318?, yes but a lot of sacrifices will need to be made.
If I recall, there are two versions of the A230
the V8 A230 has close ratios of 2.55-1.49-1.00
and the slanty has wide ratios of 3.08-1.70-1.00
the slanty has a deep low, not suited to a 4.10 rear gear, bur worse is the 1-2 split of .55. With the cam you are gonna need to make a 318 fly, this shift alone will bring your Rs down from 5600 to .55x5600=3100, and the 318 is dead in the water right there. My conclusion is that this is a bad combo.
The Magnum 5.2 starts off with more compression, but again, with the cam required to make 340 hp, she will only suffer a slightly less embarrassing 1-2 shift.

IMO you will need a 360/5.9 based combo, and if on a budget, then a 5.9 is the only option. I still would not keep the slanty A230. But on a budget, you will have to. And that means your 360 is gonna need some juice at 3100.
340 hp is now within reach with a smaller cam, which will help keep the pressure up, which will strengthen the power at 3100. You will still need the 4.10s to trap well. And you will need to have some headwork done and that's where the money is gonna be.

So to recap;
5.9 magnum, worked heads, 4bbl conversion, headers and fat duals, HO fuel pump, new cam, and timing chain set, 4.10 SG/ maybe 3.91s, drag tires, some suspension work. a tach, a decent ignition system to go 5800/6000, a rev-limiter, a helmet, a coolant recovery tank, and I forget what else.
My 367LA powered 68 Barracuda went 106/12.9@ 3650# with 3.55s and an A833. I used a 270/276/110 FTH Hughes cam with .050s of 223/230 and lifts of .538/.549
This is a dynomite street combo, but as you can see it barely made it into the 12s.
If I can; my best advice is to set it up for the street, maybe a slight bias towards the Eighth-Mile, and forget about the Quarter. It takes completely different, NOT STREET FRIENDLY rear gears, between these two contests, and a completely different, also not street friendly, cam. A really good street combo will do reasonably well in the Eighth,with just tires and maybe suspension changes.
Happy HotRodding

Edit;
the A833od trans has ratios of 309-1.67-1.00-.73od very similar to the A230slanty, making 3.91/4.10s more bearable. And these boxes I have found for 50 to 75 bucks at swap meets. I ran one for awhile with the 223cam, it sucked.