Mines an 864, running a 5" cylinder. Unless you have the high capacity pump I'm thinking a 6 1/2" cylinder will be mighty slow. In all honesty I could have went with a 4" cylinder instead of the 5" and most likely would have been fine. I have a pressure relief valve installed and a pressure gauge in plain sight, about the only way I can blow the pressure relief valve is on a VERY gnarly piece of wood or trying to shear a piece instead of splitting it. Only time I've had the high pressure relief blow was on some very nasty walnut, flipped it from end to end and it split like a hot knife thru butter.
To set the relief I grapped a rather large chunk of burr oak and tried to shear it, when I seen everything start to flex I noted the pressure on the gauge and set it a few hundred pounds under that.
If you can find it, go with a few pieces of heavy C channel instead of an I or H beam. With the C channel I spaced the channels about five inches apart so you con look between them to see exactly where you're wedge is when splitting the more common size of wood. I used two pieces of C18.75x10 which is about the heaviest you can get without special ordering it. My stuff came from ALRO steel as drops so didn't pay near the normal price. The C is channel and the 18.75 is pounds per foot, 10 is the height.