Hey Chris, have you decided on anything further? I am really looking forward to your project. I can't believe how many you have to choose from.
I really like your idea of utilizing the original frame from one of the wagons. That's what I did and it worked out great. No doubt a lot of work, but it looks right when it's all done. And they are really pretty well built frames. I would go with the youngest one you have there. I cut all the original brackets off it and boxed it in. Then welded on aftermarket spring hangers. I went with XJ springs in the rear and YJ springs in the front. BDS brand. And they really ride nicely. Not as good as a coil suspension of course, but simplicity rules in my book.
I think the 4.0, auto or manual trans, and 231 is a fantastic option. The firewall mods would not be that difficult. Although, I just removed the entire firewall and built my own. It's much easier than welding up all the holes, cracks, and whatever else in the original in my opinion. Either way works though. You will definitely need an SYE kit for the 231 though.
And about the noise; I remember my first wagon being absolutely ridiculously loud with squeaks and rattles. So this time I had the under side and interior sprayed with Vortex and it really makes a difference. Not one rattle or squeak. It is tight and much quieter. Although, not quiet enough yet. I am going to buy high quality automotive insulation and carpet to take it to the next level.
Yea it's old. Yea it's a lot of work. But in the end, you will have a **************' rig that is so rare these days. And I just don't see many well built wagons anymore. There are a lot of 2WD and more streetrod-type wagons, but not well-built 4WD wagons that are really useful for camping, expeditions, and back country travel. My first suggestion is to ditch every single component that came original on it. Don't use them at all. They are simply too old and will drive you crazy and it's just not safe these days with the speeds we drive at. Pedals, brakes, axles, steering components, wiring, drivetrain... ditch it all. Just my opinion.
I hope you go through with it. I know my build was overwhelming sometimes, but I am SO glad I stuck with it. This weekend my wife and I drove it up to Garden Valley and at one point we parked and went in to check out some antique shops and whatever. And when we came out it really shocked me how much it stood out from all the other vehicles. It's just so much different than anything you see today. It's totally worth all the late nights.
I would like to take a drive up there to see your project sometime this summer or this fall.