Well, all this information is great, and I understand more than I did about modifications that can be done. But, it's also discouraging. The FI sounded like a good idea until the issue of electric fuel pump emerged. At this point, it seems that it would be very easy for me to go down rabbit holes of modifications of ever increasing complexity and expense. In addition to the cost of these expensive mods, I would most likely have to have a mechanic do most of the work. And, to find a mechanic who actually even knows what a "Slant Six" is, might be a challenge. I think I would rather focus on what modifications might be closer to my skills, and budget. Then again, I could just leave it alone and drive it like it is. Kind of disappointing, but a lot less work and money. Working with wood and on houses is a lot easier. Oh well.
Fuel injection is great, but at the end of the day you'll be $2k lighter and still have a wheezy little turd between the fenders. Your wagon is heavier than my sedan and that motor realistically probably only produces 90 horsepower. You can add the typical bolt on and probably be somewhere in the 120 horsepower range. After you tear into it to increase the compression, do some head work and change the cam you'll be somewhere in the neighborhood of 175 horsepower and still not produce the torque a factory built 318 will. Lots of expense for very little gain.
Honestly, if you're not willing to commit to a V8 swap I would leave it all be.