Could be a number of things. Whats your gear ratio? I doubt thats gunna bean issue with your 31's.
You have a 75 Bronco which means you have vented gas lines that enter the cab just behind the driverseat or just in front of the wheel well.
You can try to cap them off but you can get vapor lock and starve the motor of fuel. Most add a vented gas cap to help.
From the sounds of it you have more issues than this.
If its not kicking down try adjusting the linkage so it kicks down sooner.
Check or timming, if its off you will get all kinds of unburnt gas smells. Do you have exhaust leaks? With the short Bronco exhause fumes want to travel back up and around it and will fume you out. Good gaskets will help. Make sure the firewall, trans tunnel do not have air leaks.