Slicks = even more contact patch on the road/ more parasitic drag that comes with the traction.
Road bikes have skinny tires for efficiency.
What you have on your hands is a street/ strip car that is suited for the occasional cruising, the way it is.
Even if you set up a vacuum gauge and minded your driving habits, like anticipate traffic, don't drive aggressive and anything else you can do to leave the gas and brake pedal use to an absolute minimum, without a hood, without tuning with an O2 sensor in the header on idle, cruise and WOT positions, you are looking at a gain of about 1mpg by monitoring, alone, given that you have done your homework, don't brake often, don't accelerate hard, etc.
I ride a train to work every day. I just got a cherokee that I'll be doing some mods to get me into higher mpg range and guess what I'll be doing? Riding the train to work every day. It costs me $68 even, every single month, like clockwork. I don't care what kind of transportation there is on the road, nothing will come close to that. Not even a scooter @100mpg+, after insurance that will cover accident/ hospital cost reasonably. Forget winter driving. It murders everything by a long shot at that price.
I know that not everyone can do that, but my point is, unless you are willing to get something else, or make some reasonable sacrifices with your current setup, in order to keep performance and get MPG, you won't see it. You are going to have to get out of old school tech/ mind frame with your entire drivetrain.
I think you need gears, more than anything. Overdrive or at least a good converter and a good, wide band o2 sensor, weld a bung into your exhaust as close to the runners as you can, check AFR on a reader, log your idle, cruise (high vacuum) and wide open throttle, as well as the transition. You will need a friend monitoring and recording while you drive.
This will tell you what kind of metering your carb will need with everything, including accelerator pump shot.
Reading plugs can't really be done, once you're close, anymore, because of the increased amount of ethanol used in fuels, today. If you are close, but not on, it won't look warm. It will look lean, but too lean is also bad, so you really need to look on ebay, summit, amazon, etc. and find an O2 AFR reader kit with a wide band o2 that is used only when tuning and capped, to get the job done right, that suits your budget and needs.