RIP 340 Onto Magnum swap

Sounds like you're half way there already :)

The crank sensor is part of the flexplate, but it's possible to adapt the A833 flywheel to the Magnum EFI flexplate. Have a look at 5:33, if you don't want to watch the whole video:

I've never looked, but I've come across people stating this several times and never actually seen it work, and a flywheel isn't something I want to work some half-assery on.
There are (or at least, used to be) Magnum-EFI-Ready flywheels on the market that were competitively priced. American Powertrain was one but I don't see it on their site any more.