BS45,
If there's no problem with the range safety rules, I would just give it a try - I wasn't the only 45-70 shooter in Bisley (maybe one of the worst [performing though, but that's another thing).  I would, however, load it to max. velocity you can get. (FWIW, I used a 520 gr bullet with 70grs of Swiss 1 1/2 Fg, and I don't believe I have to blame the load for my bad performance...)  However, next time, I'll try to up the load a bit more (75grs or even more, as much as I can get in and still shoot accurately), as any gain in velocity is bound to show up over there as less vertical spread, as long as your ES remains respectable.
But if range safety rules allow you to, by all means, do so - there's no other way to learn this game.

BS45,
When I started out shooting LR it was with a Shiloh #1 Sporter in 45-70.  It worked OK at 800 and 900 yds but could not quite keep up with the 90's at 1000 yds.  It seemed to "run out of gas" beyond 900 yds.  Before I sold it off and built a LR rifle on a Rolling Block, I tried a 2 diameter bullet which allowed me to seat the bullet out of the case a good ways to get more capacity in the cartridge.  This was before Swiss powder was available.  I'd used Goex CTG powder and was able to get about 85 gr. into the case with a lot of compression.  It worked almost as well as my purpose built LR rifle that used a 45-70 case along with .300" of freebore.  This also allowed me to load 85-87 gr of the Goex powder into the case with a single diameter bullet.   
Today, I use a 45-90 case with additional freebore to get 90 - 92 gr of Swiss 1.5 behind a Jones 545 gr. bullet.  Good luck and keep us posted on any long range oppertunities in PA, as it not a long drive for me to get there.   

P.S. there are long range matches held at Quantico USMC base 2 times each year.  The next one is the weekend of Aug. 27, 28.           Frank M.