Lynda Boone Burton candy recipes

The fudge recipe you posted has an error in it. You have the temperature for sea level and high altitude. However, for high altitude the temperature should always be lower than sea level. It doesn't matter if your thermometer is off a little bit, as long as you always test it right before making the candy. Put it into a pan of water, bring the water to a boil and see what the temperature is. For example, most of the time water boils at 203 here at my home. (5,000 ft). So when I make candy, I subtract 9 degrees from what it says to cook to. (Since 212-203=9) So for my walnut caramels that say to cook to 250, I only cook to 241. If they go all the way to 250 they're so hard you can't eat them. However, some of Lynda's recipes were adjusted for high altitude already. I'll post my copies with the sea level temps so people can adjust from there. 
For the marble, you will need AT LEAST a two foot by two foot slab--mine is around 20 inches and the candy fills nearly the entire thing. The thicker the marble, the better it cools. I have one that's only a half inch thick and it's always my last choice because the 3/4 inch slabs work so much better. If you don't have a marble, you can probably get one from a masonry store for the best sizes and price. Buying one at a kitchen supply store is silly. They're too small and too expensive. I bought mine from Buehner Block in SLC because they do a lot of fireplaces in marble for people and so they had a lot of odds and ends that weren't good for anything else.

Lynda Boone's Chocolate Fudge
4 cups sugar
3/4 cups water
1/4 tsp salt
4 squares Baker’s brand unsweetened chocolate, shaved
1/4 cup butter
3/4 cup evaporated milk
Heat salt, water and butter until butter is melted. Add chocolate; heat over low heat until melted. Stir until the consistency of gravy paste. Add evaporated milk and cook and stir until mixture thickens. Add sugar, mix well. When the mixture is nearing a boil, wash down the sides and cover until boil is reached. Uncover and add thermometer. It will rapidly go to about 218º F. (at 5000 ft) and then stay there for some time. Cook to 224ºF. at 5000 ft (232º at sea level) without stirring. Pour out on cold marble, let cool a couple of minutes. Stir on marble. When mixture is about to set, stir in 1 tsp vanilla and 1 cup nuts if desired.

Lynda Boone's Buttermints
1 cup water; dash salt; color
3 cups sugar
1/2 cup butter, cut in small pieces
Flavoring
At a low temperature, bring water, salt, and color to a boil. Add 3 cups sugar, stir till dissolved. Wash down sides, cover for 3 minutes. Take off lid, add thermometer. Cook to 242ºF (250º at sea level; softball). Add butter. Cook to 252ºF (260º). Pour onto cold, buttered marble. Let cool a bit, turning in hard edges. Pull to let air in. Add flavoring. When it just starts to lose its gloss between pulls, shape into ropes and cut into pieces.

Also, please note that the fudge uses evaporated milk as opposed to sweetened condensed milk, which is a completely different thing. Eagle brand may have been Lynda's brand of choice, but that is sweetened condensed, so it would have been for the caramels, not the fudge. The reason for using an evaporated milk in candy making is to shorten your cooking time while still adding some of the caramelized flavor.

I'm pretty sure my marble pieces aren't that big, but I think I know someone who can 'hook me up' with some. How long does it need to be in the freezer? It seems like it could be big and heavy to deal with.