Sounds promising.
I love edibles. My fav. is the orange and almonds brownies - they are fucking awesome if done right.
I find it pretty hard to guess how strong a piece would be before sampling it. I did some calculations while making some canna-chocolate fudge and it turned out pretty much as the calculations shown but when its more ingredients it becomes tricky.
5 hours of boiling sounds great. I usually do 3 hours in a double boiler and wash the butter after it chills. That way I can shape it with ease.
What recipe did you use to make the mars bars?