manual trans with 5.9 magnum

If you want to keep the EFI and the manual trans, you will need an aftermarket flywheel. The flywheel has the crank tone ring notches the PCM uses to know the crank rotation and the stock Magnum flywheel is too large for a car bellhousing. I know of one guy that had one turned down to fit the car starter ring, but seems hit and miss as to getting a shop to do this.

You will also need a different bellhousing to bolt up to your 3 speed. This bellhousing will require a notch to be added for the crank position sensor as well.

In the end, it might be easier all around to find a small block 904/727 and use that if all you want is a cruiser. Unless you really want to use a stick.