Two notes:
1) I used pyrocat HD for the interposiive as well as for the final enlarged neg. I found some x-ray film which is still available, for a very reasonable price, with the emulsion coated only on one side (normal x-ray films are coated on two sides) which works very well for the interpositive (using normal films means total darkness, and normally the necessity of a shadow mask done also from APHS. The x-ray film demanded the addition of some vitamin c in the developer, which I also added for the development of the final neg).
Development of the enlarged neg was normally for 45-55 min (!) in 1+500. A Jobo would be handy here, which I don't have, but I I covered the dish and meved in and out to rock every 3 min, after the first 10 min continuous rocking.
The problem: the developed neg will probably have some layer from the alkali on it which has to be removed by whiping it of with cotton dipped in citric acid.
2) However, all in all I would rather recommend Liam Lawless' reversal method, as it saves the interpositiv. The problem here: if the original neg is too contrasty, the highlights will lack contrast. Two solutions: a) two enlarged negs, one for the shadows and midtones, one for the highlights, taped together in registration. b) develop with half the recommended concentration for a longer time, initially 12 minutes, be careful the developer gets fast exhausted this way.