Does anyone know if it's possible to reload Barnes 500gr solids in the 45-70 Encore, or is the bullet to long??

It's certainly possible but may cut significantly into the powder charge you can use. You also may not be able to get enough velocity to stabilize the bullet adequately for accuracy since I suspect the Barnes solid is probably a bit longer than more conventional jacketed lead bullets.

The Barnes manual only lists 45-70 loads with up to 400 gr. bullets. You could take loads for 500 gr. bullets listed in the Hornady manual for your starting point if you really want to try this.

I've tried 500 gr. Barnes bullets in a 458 Lott and besides being quite expensive, were quite a bit less accurate than Woodleigh and Hornady 500 gr. bullets.

Barnes doesn’t list loads for the 500-grain bullets in the 45-70 or 450 Marlin. Just for grins I asked Quickload about it. Quickload lists the total volume of the 45-70 case as 78.99 grains/H20. The seated 500-grain Barnes bullet takes up 45.576 of that space. Not much room left for powder.

Like Grumulkin said, Hornady lists a load for their 500-grain bullets in a Ruger No 1. Hornady’s bullets are lead core and Barnes is an all copper/zinc alloy, not exactly an apples to apples comparison.