You can now buy FP type metal can capacitors at places like Antique Electronic Supply. They will fit perfectly and the ones I use in that position are generally rated at 475 volts DC. If you are replacing something like a two section @ 40 uf capacitor, you can use the quad section 20,20,20,20 uf @ 475 volts and just pair up sections to give you a two section @ 40 uf. As for the bias capacitor. This baby is a positive ground capacitor. You can find Nichicon 100 uf @ 100 volt capacitors that can fit under the chassis if you are creative.
My line voltage here varies from 115 - 120 volts so it is lower then what I had in upstate N.Y. in the late 60's (was 121 volts). But your line voltage could be as high as 125 volts. Also, if you bias this amp too cool (plate current on the 7189's too low) your B+ voltage at this capacitor will be higher then normal.
So biasing the amp reasonably close to 44 ma per pair of output tubes is a good thing to do. I have serviced these amps and found the bias adjusted as cool as 18 ma per pair of output tubes by the owner or someone.
Change out all those can capacitors. Just replace them with modern production cans, or modern production multi section clamp mount capacitors like those new JJ ones. The JJ's are a lot cheaper while the new FP types will retain the original look. Depends on how much looking original means to you as to whether you want to use the JJ capacitors or the more expensive FP type twist lock cans.
Another trick is to drill out the guts of the original cap and stuff the thing with some modern axle lead type single capacitors. I've done this a couple times before FP types were again available. But this is a bit labor intensive.
You should also change out the bias rectifier, coupling capacitors and all plate feed resistors. I have had best luck with metal film and metal oxide plate feed resistors. They tend to be quieter and more stable. But, I prefer carbon resistors in the signal path. You will have to try some different types of coupling capacitors. I'm older and my ears tend to roll off the highs more then a younger person (under 35 years old). So I like metalized poly caps for coupling capacitors. Ones like Kimber, Solen, Xicon, and Auricaps work for me. But I tend to suggest paper in oil types for younger people and use them in rebuilds where people ask for a softer sound. The differences really are very subtle, but they are there. My usual test is to see if you can hear the 15 Khz whine of a CRT type monitor's horizontal oscillator. If you can, chances are you'll like paper in oil caps. If not, you'll appreciate the extra sparkle of metalized film type capacitors. This is just a guideline I have adopted over the past couple years. It isn't cast in stone. Let your own ears decide.
Good luck with your amp. Those amps, when done right, sound really nice.
Oh, the schematics as published by Scott and Sams rarely are exactly what you will find in the amp. This is because Scott made constant changes to their amps during a product run.
