The picture above shows the inside of my Acoustat Red Medallion C interface with the new Solen 47uF capacitor in front. It looks like the new capacitor (bottom) will fit either in front of the circuit board on right side, or above big transformer in back. There's more space in back, but nothing convenient to support the capacitor with and a long lead would be required, and there's high voltage back there. I'm leaning toward front installation.
In front, the capacitor could rise straight from where the fuseholder is, or lie flat above the fuse and two speaker terminals. Either way space is very tight, and one concern is that the capacitor would be very close to the 50K power resistor. If the resistor gets hot it could damage the capacitor, and there is high voltage on that resistor too. I might be able to give it "just enough" space, it looks like I could give it about 5mm space. Opportunities for support are also limited, but it would be somewhat self-supporting from it's own lead, so doesn't need that much additional support. If it falls "down" in operation it simply hits the wood base of the speaker. where the interace is installed.
I could possibly bend the power resistor out of the way, bit by bit using one or two sets of longnose pliers. The danger is that I could break it off it's solder pads, or, even worse, crack the old circuit board.
Another thought crosses my mind too. I could connect the capacitor BEFORE the fuse, so the capacitor effectively bypasses the fuse for the high frequencies. The speaker would still be protected from DC and low frequencies, which is likely where any problem would be. The fuse doesn't really do a great job of protection anyway from sustained loud music, sustained loud music can burn the transformer without melting the fuse. The fuse mainly protects against amplifier failure, and also provides nuisance warnings that you are playing too loud.
I could also have the capacitor outside, either as an independent entity (with new input terminal) allowing easy changes, but clumsy and space consuming, or strapped to the case with twist tie (requires new holes in case, not easy) or glued (ugly).