[Arran]

I would say that it depends on how the original capacitors were bypassed, and also what circuit they are in. If they simply connected to new capacitors across the old, and left the old ones in circuit then I would not play it as it is. Replacement caps from the 80's are probably new enough not to be a problem in and of themselves, but if they were smart with the electrolytic filter caps then they would have disconnected everything from the old positive terminals and left only the negative, or can side, connected. You can use the negative side of the cans as  tie points for the new filter caps but you should never connect the new filters across the old. The molded paper caps, well they might be somewhat functional, some of the Solar brand ones will still work, but they are living on borrowed time, I think that they may be coupling caps and the bypass caps are the ones in the tin boxes, but if they are bypass caps between B+ and ground I would not trust them. The ones in the tin boxes are really easy to restuff, I think they are lined with paper so you don't even have to melt out any tar, just pull them open.
Regards
Arran