Flywheel bolts on 351w based 3" main cranks don't usually go thru to
oil in pan as they are blind.
With regard to the RMS did you install the seal halves with ends staggered in relation to cap face , eg 1 up, 1 down, by about 1/4" to ensure seal ends are correctly lined up?
Grimey stuff could be clutch lining material mixed with
oil.