I think you are bumping up against a value issue here.
It's true that the challenge would probably be easier to complete using Bud Light. But a pint of BL can't cost more than, what, $2.50 at the bar? That's $40 in beer. And, c'mon, what's the shirt worth, $5 maybe? Which means you are basically throwing $5 away, which brings a distinct air of flippant irresponsibility to an otherwise dignified and intelligent challenge.
Now, with the Guinness, you are probably topping out at over $4 or so, which gets you to $64 of beer. Tack on the $5 for the shirt and you make almost $20 on the deal. Now that's thinking with your head! At the end of the challenge, the whole bar will look at you laying on the floor, barely coherent and say, "Now, THERE is a smart fellow! Someone who really knows the value of a dollar!"
As an aside, for your weight, I'd definitely make it an 8 hour visit. That puts you at one beer every half hour, which seems doable. Shorter than that you probably put yourself in jeopardy of finishing the challenge and/or of living through the experience.
Overall, I'd do 2 things:
1) Maximize your profits, drink the most expensive beer they will serve you for the challenge.
2) Call a cab, you are going to be wrecked.