The valve base is a B8G, which has a centre hole with a keyway to take the splined centre locating spigot of the valve. The locating spigot is hollow and encloses the glass envelope pinch-off seal. If the centre hole in the valve base is the same as that for a B8G then it should accomodate the locating spigot containing the pinch-off seal quite easily.

For some reason the valve bases in your DAC would appear to have smaller central holes, just large enough to only accomodate the seal. If that is the case then you really have no option but to remove the aluminium case.

However before you attemp the above, I would mark the two pins flanking the spline of the valve loction spigot, so you know how to orientate the valves when fitting them inti their sockets.

I would use a jeweller's screwdriver to prise up the case where it joins the metal valve base, then with a pair of needle-nosed pliers enlarge the gap so that a pair of small side snip wire cutters could cut a slot so as to peel away the aluminium can.