pliny recipe // full boil question ?

Your best bet, if you want the beer exactly as intended, you'll have to scale it down to 2/3 - 3/4 somewhere in that range.
First off if you boil 6 1/2 or 7 gallons in a 30 qt pot for an extract beer you're gonna have a big boil over mess. Usually, a pot is measured to the brim. It's really tough to keep a rolling boil under control with only an inch or two from the top. I know from experience it can be done though, just a real pain in the ass turning heat up and down, cutting and skimming foam, it sucks but you can.
Now the second part of the question, how will it effect the beer? With less water in the boil your wort is thicker, and therefore, the will be less hop utilization. How much less, I don't know without plugging it in to a calc. But as a totally theoretical assumption without any backing, remove ~12.5% and get about ~12.5% less hop utilization. Then one more problem will be for a 90 minute boil you will likely lose around 2 gallons of water depending on other variables. So you will have to add water to come up with enough in the fermenter.

 

BeerSmith has a scaling feature you could use to help you scale down the recipe to fit your equipment. I use it a lot as I am currently only able to do full boil for 2.5 - 3 gallon batches. My pre-boil volume is around 4.5 gallons. I lose approx 1 gal per hour to evaporation, plus other miscellaneous losses (trub, sampling)

 

If you watch closely, there is no reason that you cannot boil 6 gallons in a 7.5 gallon pot. You can keep another small pot of boiling water to constantly top off to 6 gallons throughout the boil. Not ideal, but this is what I would do.

 

http://www.purplemath.com/modules/ratio2.htm

 

Fermcap works great. I had 7g in a 30 qt pot. NO boilover using fermcap.
I would not have believed it unless I saw it with my own eyes.