[Electronic M]

Once you get those parts in I would recommend temporarily removing the HV cage door, running the set over 5 min in a very dimly lit room and trying to locate the exact point of the hiss. The hiss is probably corona (the HV discharge not the virus) or arcing. HV cages collect dirt like a magnet and dirt can be conductive. Cleaning with isopropyl (rubbing) alcohol and glass cleaner will help. Some sets had a rubber insulation tire on the flyback and those sometimes become somewhat conductive and have to be peeled off and replaced with sensor safe silicone...Shango66 on youtube has some good vids on the topic (on RCA CTC17 sets IIRC).

EDIT: I'd also measure the 1000V and 800V boost supplies. Knowing if they are below (excessive load on flyback or insufficient drive), within 10% of rated( normal), or high (under loaded or overdriven) as well as control and screen grid voltages on the output tube usually gives a pretty accurate picture of where in the horizontal circuit the problem(s) lurk.
If your DMM can only read 600V (common limitation) you can get around it with this clever trick (came up with it on the spot while servicing a console in the owners home with limited equipment)...Connect meter neg to ground and measure a B+ rail in the 400-500V neighborhood and write down it's value, then connect meter negative to that B+rail you just checked and the positive meter terminal to B+ boost and add the noted value to your meter reading to get the actual boost voltage. The difference between a 450V rail and the 1KV boost rail is only 550V so a meter rated for 600V max can safely measure it.

First priority should be a good stable horizontal circuit, followed by focus, then anything needed to make the monochrome picture optimal, then fixing the color.