Have made a number of 100 yd day/night shots on coyotes and bobcats and a few daytime out to 200 yards. You pick up a little movement on windy days from springs; also if riding and stop for a quick shot sometimes have to wait a second for shocks to catch up and settle down.
Your wheel spindle and hub should do fine. I agree the boat swivels are a bit light duty for use on hunting chair.
I had an old swivel chair from the galley of a WWII liberty ship with cast steel swivel and bolted it to boat seat.
The height of seat cushion on my chair is 5' on level ground. The higher you go, the more movement will be multiplied.
One thing I forgot to mention is that you do need a seat belt if you plan to ride the chair from stand to stand as you can have a pretty rough ride on rough terrain.
Good to have hand held radios for communication w/driver, as well.
The higher you can put your braces, the less spring you will get @ the chair. You might think about running side braces to your back door hinges. I bolted angle iron to bolts holding bumper to bumper brackets and put eye bolts on the angle iron to attach chains.
Have fun building your chair. Mine has evolved over the years to include the game hoist and a spindle to hold 200 yard spool of 3/4' poly rope used to drag larger animals out of mud/water.
Regards,
hm