I've been researching doing the same thing to my 360, and it seems pretty reasonable to do except for two major headaches: The intake manifold bolts on magnum engines are vertical (vs. perpendicular to the head with LA heads), and the crankshaft position sensor.
The "beer barrel" design of the intake manifold means that the bolts
must be installed vertically -- you can't redrill the intake to accept the LA pattern, which is more perpendicular to the heads. If you're content on using the OEM intake, it means you must either redrill the LA heads for the magnum pattern (usually is done the other way around, drilling magnum heads for LA intakes), or put magnum heads on your engine. Redrilling the heads is pretty close to free if you have the tools, but you risk ruining the head. Putting magnum heads on the 340 means better flow and a higher compression ratio, but it also means you have to get new pushrods. Magnum heads oil the rocker arms through hollow pushrods, whereas the LA heads have oil passages drilled in them to oil the rockers.
Onto the next problem.
Magnum engines use two signals from the engine to determine where it is in its rotation: camshaft position and crankshaft position. Thankfully, the camshaft position sensor is actually part of the distributor, and so you can just drop a magnum distributor into an LA engine and it'll work fine for an EFI conversion. Unfortunately, chrysler put the crankshaft position sensor in the most pain in the *** location: the back of the engine.
The crank position sensor is the dangly bit just below the right cylinder head in the picture. The sensor itself reads a toothed wheel that is built into the flywheel/flexplate, and the sensor itself actually sits in the bellhousing of the transmission at the back of the motor. So far, I've really only found one cheap, easy, and simple(ish) solution to this, and that is to relocate the crank position sensor to the crank pulley. I'm sure if you talked to this gentleman here he could get you started in the right direction: [
[FOR SALE] - 36-1 Crank Trigger Wheels 318/340/360/383/440 ]. Be careful, the factory toothed wheel has eight teeth, and these have 36. You would have to talk to him to see if he could make an eight toothed wheel instead -- no promises.
But other than those two issues, it really isn't rocket surgery. You just have to get all the parts from an old magnum motor or magnum powered car, and then also install a fuel system designed for EFI. Granted, it would be a lot easier just to spend the $795 on FiTech and slap it on, but the sequential multiport injection of the magnum will get you marginally better driveability and throttle response (speaking from theory, not experience here). Honestly, if you are going to do this conversion I would almost encourage you to go with a Megasquirt ECU, as it will allow you to mix and match sensors and allow you to truly tune your engine.
