You don't solder wires to screw terminals. You use a solder lug or an eyelet lug. The lug goes under the screw and wires are soldered to the lug.
The problem with searching for replacements for old filter caps like that is expecting to find the exact value. 6000uf is not a standard value. But something like 5600 or 6800 are standard values. And if you can't find 50v, 63v is a common rating. So instead of that exact part, use something close from standard values. You may not like going from 6000 to 5600, but really, it is a 10% difference.
Caps today are much smaller physically than they used to be, so a similar value cap today should be smaller than the old one.
I just looked in Mouser, and 6800uf 50v caps with wire leads, they show 8 choices in stock, most in the $4-$5 range. They are 22mm diameter and 50mm long. With snap-in leads, they have even more selection. At 6000uf even, they have none in stock. That is the difference between standard values and odd numbers like 6000. It sure won;t hurt that amp to have 6800uf instead of 6000uf filter caps.
Same thing happens with 1980s era Peavey amps. They all used the 5000uf 55v filter caps in a can mount. A non-standard value. Can't get them. Order one from PV and they will send you a 4700uf 63v cap. And that is how you find such odd parts.