Hello Brad,
I'm picturing that gallon of Deet and a 4" paint brush sitting beside us as we work. Of course a 12 gauge with bird shot for the larger ones.
When I started gathering parts for the tri car I purchased a bunch of coil springs that were used for the forks on mini bikes. They were reasonable in price and when I got them they looked pretty good but they weren't as stiff as I wanted. In the end I used coiled porch springs. They hang from the ceiling and the swing hooks on to them. You can find them on eBay and Lowes has them as well as Home Depot I'm sure.
If you would like to try some mini bike springs let me know and I'll send you a couple to try out. I have a few.
Like I said they were a good deal.
I just went out and measured the mini bike springs. They are 5-1/2" tall, 1-7/16" across and 1/8" rod and a 3/8" space between the coils. See if you can find something like that at Ace or another hardware store to compare them or as I said let me know and I'll jet you a couple and you can try them.
The porch springs are 2' across with 1/4" rod and a 1/4" space between the coils and if I remember correctly 6" tall.
Now with our builds we had to compensate for someone sitting over the front axle. Since most of Silverbears and my friends go by nick names such as Moose, Ox and Beef and are know collectively as the Large Fellas I put some pretty heavy springs in.
If your going the cargo box route unless your carrying concrete or some such weight you may well be able to use a milder spring.
I've stood on the ones I have in mine and there is some action left for road shocks with the seat on and me acting as the passenger weight.
I think with a front end braced like the Indian Tri Car is you won't have any problems and the springs as I said will just be for soaking up the road shocks and to allow some leaning into the turns.
Again if you want the springs or any measurements let us know. It would be fun to see more tri cars built.
Steve.