Here's the basic frame 16"wide, axle will be 1"x30"w, with 3/4" step downs, 1 live end and 1 dead end for mounting a moto disc brake system, Hope to get it through the side gate...
Probably won't fit your through the gate requirement, but my guess is that it would be a better build. Add springs, the bicycle front fork assembly. I would also look for a way to use rims and tires that would be similar to bicycle fat tires. That could make a good off road vehicle. Don't forget a roll cage and seatbelt harness.
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That's what I ordered off eBay with 4" x 4 bolt plus a 30" axle, my inside alum frame will be 16", now remember this is for around town fun at about 30mph.