I worked up some loads in my pre 23 ODM 6 1/2" . 7.5grs 800X / Fed 100 with LY 358429 173gr SWC ran 1223fps ES 64 SD 14 & had no problem holding the 10 ring on a B-8 @ 25yds like the 2400 loads ( same Keith bullet ) accuracy & spreads did not tighten until the 1200 + fps range ( 12.5grs same bullet ran 1235fps ). H4227 @ 12.5grs WSPM cap same bullet runs 1150ish & tighter groups . All hotter than the more reasonable 7.5grs SR4756 Fed 100 w/ Keith 358429 @ 1100fps , 8.0grs with a RCBS 158 or 358156 . 5.5grs Unique is +P w/ a 158 , 7.0grs Unique w/ a 158 is 357 mag territory . My gun was made in 1953 , OPs are even older . ? is how much do you want to push an irreplacable .
Case capacity & pressure limit make efficent powder choices somewhat limited for these type loads . Slower powders don't reach their preferred pressures within the 1150fps limit .