Help me get into the 12's for cheap

If you are going to start with a junkyard short block, start with the Magnum engine for a couple of reasons. If you don't plan to rebuild the short block, the Magnum will more likely have less wear and tear and better tolerances.

If you pulled the heads on the LA 360 and found out the pistons are .088 down in the hole, what are you going to do? The Magnum should be much better in this regard and be a better foundation to build on.

$250 for the short block (make sure you get the flexplate with it)
$100 for oil pan and pickup (you can go used to save $$$)
$850 EQ heads (order with the springs you need for your cam)
$350 custom ground roller (you can go with a regrind here to save $$$)
$000 reuse rockers
$ 85 new pushrods
$ 75 shift kit for trans (you can lookup how to do this without a kit to save $$$)
$ 60 thin head gaskets
$600 torque converter (you get what you pay for here)

You want a good converter for a couple of reasons. With a good converter you can have a higher stall at the track and still retain good street manners. You should be able to have 3200 - 3500 stall at the track and be able to drive around on the street normally without melting the transmission.

Quick example: I once had a car that had an after market torque converter that cost close to $300. I replaced it with a custom that cost $525 at the time and went from 13.20s with a 2.02 60ft to 12.30s with a 1.58 60ft. on MT ET Drag radial 235-60R15.

The 2 biggest things that control how an automatic car performs is the cam and the converter. Go custom on both and know for sure they are a matched set.