This is a classic example of "they don't make them like they used to". You have discovered that there is a very good reason they don't make them like they used to! There is a two terminal strip to the left of the power transformer and more or less behind the power switch. One terminal has a yellow-looking high voltage wire from an output transformer connected to it. The other terminal has nothing connected to it and is bolted to the chassis. That would be a place you could try attaching the green power safety ground wire, as it looks like it should reach. UL approved connection requires a dedicated screw, solder tab and lockwashers, which is what we do in our kits. But attaching the ground wire to that empty terminal will yield the same result.
Another way we don't make them like they used to is we isolate all the iron from the chassis with insulated washers and run a drain wire from each transformer or choke body (metal) to the same safety ground point. This reduces eddy currents in the chassis that can create hum and buzz and still keeps the amp safe in case of a short from a transformer winding to the lams and end bells. That's a lot more work to retrofit than properly attaching the safety ground wire to the chassis.
I can't tell for sure, but it also looks like old filter cap is completely disconnected? If so, that is probably fine. Otherwise there might be some concerns about ground loops there, too.
