I have 2 1891s, an original with chrome furniture and one that was sporterized when I got it. It now is on it's second barrel.
You can use the 123 or 125gr. .310" bullet and 2400 or Unique for a reduced load. H4895 can be used at 60% of the max listed load also.
I have used many different bullets over the years, including the 32 caliber pistol bullets with fast powders like Unique and Red Dot.
In the July Shooting times Lane Pearce has an article on handloading for the 7.65 and uses the 123 and 125 gr bullets in full power loads. He does write about forming cases from the 30-06, but and easier method than what he uses it to buy the trim die from RCBS. Lube, form, cut with a hacksaw, deburr, full length and load.
My sporter will group the Hornady 150gr with a full load of surplus 4895 into less than 2" at 200 yards. I have taken quite a few head of game with this gun. Usually use the 180gr. Rem roundnose and AA2520 for elk.
I don't think the extracter in a '91 will work well with the 7.62X39 as it is much smaller than the '98.