looking at the numbers length 1.76 Head dia .581 rim thickness .071 rim dia .662
BAC sells this made out of 50-90 sharps length 2.5 head dia .562 .020 too small rim thickness .062 .009 thin rim dia .663 
Weatherby 378  mag length 2.90 head dia .582 Cut belt off and add rim I have played with this for 44-77 Sharps with win mag brass. 
ther is other brass but it is all hard to find. 
first thing I don't have is dies 

Actually, they didn't.  The best and most successful LR BP cartridges actually we're BN's, and continued in use long after the US lost interest in LR shooting. .461 Gibbs (both no.1 and no.2), no.2 Musket.
Yes, LR BP shooting was also done in other countries...
And 40-70 What Cheer hardly qualifies as an early BN cartridge.

True, but a Gibbs-Metford or a Deeley & Edge don't.  And still they used BN cartridges.

Dies, Check CH4D - if they don't have it, they will make it for you...  Alternatively, check dimensions of similar cartridges, maybe you can use a more easily available die for case forming.
Like I used a no.2 Musket die for forming .45 Turkish.  Afterwards necksizing only with a bushing necksizer, worked very well.
Is this for an original rifle?  If so, pics please!

There's a way to do it, once you know it's easy, Ill try to explain.
1. Put the rifle on a stable rest, leaving the lever free to move
2. Close lever and make sure the sear is in fired/uncocked position (tilted fully forward)
3. Take your breechblock and check whether the longest striker slot is lined up below, for the sear lever.
4. Put the breechblock in front end slightly down, breech knuckle still above thee receiver.
5. The delicate step, and will take both hands:  press the breechblock downwards and slightly forwards, against the striker spring tension, while lightly working the lever up and down - do NOT lower it too much, the sear should not go to cocked position.  This means removing the block, repositioning the sear, and starting over.

This should allow the breechblock to click in with moderate pressure, but you need the right breechblock angle.  Too steep or not steep enough, and it won't work.  A 1st time requires some trial and error.

Once you know the technique and your rifle, you'll do it blindfolded - but the 1st time is difficult.