get ready to do some radiator moving and firewall work if you go with a modern 6. The Plymouth engine is only 2 feet long. Most modern 6's including the /6 are 30 plus. The common trick with v8 to older mopar is to mout the engine a couple inches toward the pass side. This gives you clearence for the steering box. If you are still considering a sub look into an early 2wd Dakota. Then you get brakes steering and mounts for any Mopar engine. But you already have IFS, all you need to do for an upgrade is to move the front shock mounts for the frame and buy one of the several disc brake adapters that are out there. Don;t make it harder than it needs to be.