Just sold a 435 to a member, recently. I did the synthetic seal conversion. Keep the factory spring, if it's in good shape. The 435 is a short stroke "magnum" . It was billed as a "magnum", in it's time, but, was overshadowed by the advent of the Diana 34. On a good day, the 435 will only get about 650-700fps. If you find that you need a new spring, I would check with ARH. You'll need to have one custom made, as no current production spring will interchange.
Dan,What was the measures for the OEM HW50S spring you used?I can't remember what is in my D35. I think its the same as yours- a very thin .118 or so wire with maybe 38 coils. Also, the piston tube ID was way wider than the springs OD. I think the spring OD was .76". Knowing the ODs of the springs you used would help too.Also, what about that blind keeper screw to remove the leather piston seal? I don't know anything about that and mine definitely needs replaced. Someone at some time thought it would be fun to try to shoot steel shot with my rifle and several of those shot found their way into the compression chamber and made a mess!! I picked at least seven out of the leather seal where they had become embedded....But I'm sure the spring in my rifle is a bit saggy and stands to be replaced too. I'm only getting at best 500 fps with 8 gr pellets if I remember right.Between the beat up piston seal and the old spring, I suppose I can't expect much more. Before I cleaned and rebuilt the rifle I only got about 190 fps.