[gregb]

OK, the two positives are going to the 25Z5 cathodes so that leaves just the other two leads which will be the negative leads. In the radio you should be able to locate the two original leads that were connected else where, not the 25Z5 tube. Those are the two negative leads you need. If the schematic is true to the actual construction of the radio the input to the choke is also connected to the on/off switch. One of the original negative leads should be connected to this point, check with your ohm meter, should read zero ohms between the correct lead and the on/off switch. This is where the negative for the 20uf cap goes. The other negative for the 12uf cap goes to where the remaining lead goes.
Hope this helps

Gregb