'98 360 differences..

I'd do some searches for info on a Magnum swap.

The imbalance is different on those motors so you'll need to have the proper balancer and flywheel for a 5.9 rather than a 360.

The heads on those crack often and early so I'd double check that before getting started with it.

Motor mounts should work with any 360 mount, but if you're using the standard "comp" style headers, you will probably have to grind the driver's side truck engine mount entirely off of the thing to get it to work. Test fit headers outside of the car!

There's also minimal grinding to get the starter to fit. Best bet is to fit this out of the car if possible.

There are some gymnastics involved in getting a mechanical fuel pump to work if you are using the stock Magnum camshaft.

The valvetrain is entirely different. At a minimum, You must use oil through lifters.

There's a lot of good information here:
http://www.popularhotrodding.com/tech/0507phr_mopar_magnum_engine_specs/index.html