I would consider drilling through a nice, heavy aluminum plate with a tapered opening so the bolt will be even with the top. Then using a punch to mark the openings onto the baseplate. Hopefully you can go metal to metal. Then drill the metal base to match. Avoid messing with the existing openings. I'd probably use a lock nut underneath so it won't rattle loose.
I suppose you could tap the base for a thread if you wished. But whatever method you use, I'd be very precise about it. If you use a pan head bolt, you can drill a slightly oversized hole in the baseplate, and then drill with a larger bit to make a flat recess. This will allow a little wiggle room if you don't get the holes just right. Here's a drawing of that approach.
Hi Len, that router had an optional dedicated table, to which it was mounted by the rods you mention. It was a very precise system. It also had an extended depth-adjustment rod, which screwed onto one of the depth-stop adjustment bolts on the turret (which you have removed), so that depth of cut could be very finely adjusted when mounted in the table.
The build quality was better than most of what one can get today, so I doubt that there would be a problem drilling through the base plate - I did so on a Hitachi because I could not afford an Elu 45 yrs ago, and have never had a problem.
However, if you are concerned, you could use the two holes for the guide bushing template, on either side of the opening. Get high-tensile bolts that go right through the base, and use Nylock nuts on the motor side of the base plate, as Tom suggests. You will note that the holes are located in reinforcing ribs, on the motor side.
If you are concerned that that arrangement is too flimsy, you could use the two bolts to position the router and prevent lateral movement, and support the base further with a few Z-plates (such as are used to attach a table-top to the table frame/apron), placed around the edge of the router base and screwed into the table top from below.
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