The original 1970 SVT schematic lists 695V B+ for the unit that used 6550 power tubes. Looking at my personal notes I have confirmed B+ readings in that range on SVTs with 120VAC line input. Since it's common for line voltage to exceed 120V you could easily reach the 730V you measured. There are probably lots of opinions about how well modern tubes could handle that B+ but it is common for musical instrument amp designs to run tubs beyond their published ratings.
One additional factor is that you are using only 4 of the standard 6 SVT power tubes. With a stock spec power transformer you will therefore have higher voltage due to the lower loading on the power supply.