"because I'm converting the kart to a yard kart"
It gives me shivers anytime I hear making a yard kart.
Are you making it out of a cage kart or a flat kart?
The down tubes on a store bought yard kart are not there to strengthen the kart.
There there to help prevent a kids head from becoming crushed or cut off. Please, make sure you don't let your kid or any kid ride around in a yard kart, without the protection of something to stop it if it goes under something low. please.
Running under the back of a pickup, under a pick nick table or low branch is enough to kill a kid. All the head gear in the world is no good against a broken neck.