In no particular order:
Q4,6 are limiters, not drivers. If you removed them the amp would still function. But don't
If Q4,6 appear shorted, they are more or less parallel with Q5,7, the tube drivers. SO if Q5,7 are shorted, they can make Q4,6 appear so. test them again with the bad large transistor removed.
Where did you find the 6465s?
Find out what is wrong with the amp first before you think about modifying it.
All voltages assume high power mode. I would not assume any low power values.
The MJ4247 is an 8 amp transistor, I don't think current is an issue. On the other hand it is only rated 120v. In my world I would likely manipulate an MJE15032 in its place - like Music Man did. It is a 250v 8 amp part.
test those 6L6s in another amp, not a tube tester. testers can verify a bad tube, but cannot tell you a tube is good.
Pull the power tubes, set to high power. What voltages are around the socket? Got +15v on all the pin 5? Got 500v or so on all pins 3 and 4? Any pin 8s shorted to ground?
Your driver transistors have 3 ohm 5w ballast resistors in their emitter ckt. Are they healthy?
Once you have verified the 6L6s will work in another amp. Pick ONE tube and stick it in one socket. Does the amp power up? If so, move that tube down one socket, same test? Check all four sockets. Now pick one socket and try each of the four tubes in it. A lot of cross verifying, but you want the amp to work.