What are the rifling specs?  Overall length of each bullet?

Ken,
You will not be disappointed with 10 gr Unique with the 395 gr bull at 100. I've shot .62 groups with that load breech seated at 100. The 525 gr bullet, I can't say. I think it will do well, too. The bullet I used was a NEI spitzer that weighted 430 in 20/1 and 409 with 2% tin and WW. Recoil is very light it shoots at 972 fps. I have a killer duplex (kills on both ends) smokeless load that goes about 1430-50 fps, as I remember.

Frank

Boys you are both right.  

My experience with the 45/70 and 10 of Unique started at the Fairfax R&G club Ram bash 20 years ago. It was the winning load in Trapdoors and I think held the record for Rams hit at 200 yards.  I was always afraid of it however Double charging a Trapdoor is not a good thing spent a lot of time double and triple checking. It never shot well for me either.  I gave up Smokeless Trapdoor loads and went to the 38/55 & 32/40 about 1993  Since only shoot my Trapdoors with Black Powder

Bring it up to today and We shoot that same match at our Club. Spotting other shooters I often see high and low shots they can't explain, most using light charges of pistol powder.  Working up loads for a Cowboy Silhouette 32/20 Lever gun with 4.5 of Unique found the same thing high and low shots, This time pinned it down with bench testing.  Mount the gun muzzle down it's low raise the muzzle and put it on the bags it's high. Powder position is key to getting good performance with light loads of Unique

You could improve the elevation inconsistency by adding Wads but then the Chamber ringing possibilities come up.  Given all the trouble checking for double charges worrying about how the gun is mounted and all Unique for 45/70 loads is not worth the trouble.  Thinking back I suspect my problem with Unique and performance was the way I was handling the rifle

4759 can show some sensitivity to how the gun is handled but not as much as other less than case filling powders.  Best practice is to handle the gun same way every time no matter what powder you use.

Negative on 4759 is how it measures. I throw mine in a Belding and Mull and trust it thrown not weighed. Most measures won't throw consistent charges though and weighing is best.

To throw another powder out there I think but don't know for sure 4227 will give the same performance as 4759 in 45/70's and measure better. It does so in my 38/55 no real difference in how it shoots

Boats

I have found that 19 to 23 gr of IMR 4759 works great with a 420 gr bullet, 22 gr works great with a 500 gr bullet, recoil is noticable with the heavier bullet. Also had good luck with Accurate Arms 5744, 23 gr with 420 gr bullet. I have a number of 45-70s, Sharps and trapdoors, and these loads work very well in all of them. Joe S

Using the 500+ grain bullets is overkill for 100/200 yards--they were designed for long range.  And they fare better with faster twists.  But in the right rifle they can be accurate. 
My first couple years in ASSRA I used a Browning B-78 light barrel hunting style 45-70. (With a sporter-weight fast-twist factory Badger barrel----long story) I used the 500 grain nosepour Lyman-Matthews bullet, breech seated, over 25+ gr 5744.  Off bench-rest bags the heavy-bullet/heavy-load recoil gets your attention, and firing a couple hundred rounds over a match weekend verges on insanity.  its not a practice I'd recommend.
BUT . . . . .  it was the only single shot I owned at the time   AND toward the end of the first season I was shooting low 240 scores on the Schoyen target-----I've never scored that well reliably with my other guns.  The etna green winds were a non-issue.

you might rat out Matthews books on the 45-70 he has a lot of loading data.  Its been a while since I looked at them and they tend to be a bit BPCR and hunting oriented, but the 200 yard "chicken loads" might help

Ken,
These are my 45/70 groups, shot with a 1878 Sharps, 31" Douglas Benchrester barrel (1.13 straight octagon). All BSed loads with a standard 45/70 chamber and a 430 gr NEI non tapered spitzer. All shot at 100 yards.

Left to right,
25 gr 296, F210, 1.2 group
15 gr 296, 27.5 4831 (duplex), F210, .60 group
10 gr 296, 75 gr (bulk) FG, F210, .80 group (3 in .20)
10 gr Unique, F150, .72 group
 
Frank