Tube power supply capacitors replacement
Due to R7 and R8, you can increase the size of C8 and C9 without an issue. But I would not increase the size of C3 that much.  As you increase the size of C3, the diodes will be conducting for a shorter and shorter time per cycle, yet the power being supplied to the output will not vary -- so the current through the diodes (while the are conducting) must be higher.  Also, as the commenters pointed out, there is no need at the first stage of filtering to stress the diodes that much.  For the sake of reliability, I would stick with the 100 uFd specified for C3