I don't have any personal experience, but here are my thoughts from reading:
Check out the FAQ:
https://forum.ih8mud.com/40-55-seri...ions-f-q-updated-28-jul-09-a.html#post1801878
https://forum.ih8mud.com/40-55-seri...ions-f-q-updated-28-jul-09-a.html#post1801879
I would also search "SM420" and see what other hits you get. Read, read, read.
On the engine swap, it would seem to me that to get the dimensions set up correctly you would need to have the tub at least sitting on the body mounts. Take the tranny hump off so you can see where your shifters will line up. Install the radiator, fan (decide if you're going electric or mechanical) so you can determine your up-front clearance. Think about your exhaust routing. See if the brake booster will interfere with anything (on a 350, not usually a problem). Think about your steering shaft/box and how that's going to play in. Check out AA's website for engine mounts etc. Measure, measure, measure and become happy with how it's going to look. Tack it in, check it again, then burn it in when you're ready. You'll be getting a lot of use out of the engine hoist.
On the springs, flipping front, rear, or both depends on what wheelbase you want, and if you need any extra distance to clear your drivetrain. You should also think about possibly using longer springs (FJ55, FJ60 or Chevy) if you want to change the ride/load capacity. Are you planning on doing a shackle reversal? spring over? In any case you'll have to think about driveshaft mods, whether your pinion angle is going to change and have to be corrected, anti-wrap bar (for springover), and for sure make sure your front driveshaft has a long enough slip to not separate, but also so it doesn't grenade your t-case from not having enough slip.
Good luck.