Well, I'm in no way an expert on the Contender, but I know it's rated at 28k psi on the frame. That means basically anything in the rifle data that doesn't exceed that would be ok, if you can stand the recoil. I believe the trap door loads max out at 28k, so stating that, the load notes from Hornady's FTX should all be available to you, although they are somewhat limited in Powder selection:
http://www.hornady.com/assets/files/ftx_load_data/45-70_trapdoor_ftx.pdfI do believe that most of the bottom tier loads from the Marlin Load notes for FTX are also starting around that level. Unfortunately, Alliant's website isn't coming up for me to cross check where the loads cross, so you may have to do more research. Just keep them under 28K and per Thompson you should be ok. Most likely, you will want less recoil than that anyway. I have shot .357 Maximum in a contender as well as .410 slug/shot loads, so I know those kick like crazy.
I wanted to look at where my Reloader7 loads come in as that is my exclusively used powder for this caliber. Clean burning and good variability in .45-70 from 34gr. (trap door level) to 52gr. which push really hard and will rock me back on my heels standing up. I also like it's slow burn rate as it pushes more than kicks. In your handgun though, you could easily start it at 34 gr. and see how it does for you, although having a chrony might help you since your barrel length isn't going to be represented in most of the data.
http://www.hornady.com/assets/files/ftx_load_data/45-70_1895-marlin_ftx.pdfGood luck to you.
