I planted a clematis Montana over a Leylandii stump. I left it at about 6' with as many spurs on its trunk as possible and encased it with trellis. I also used 3 galvanised poles as a wigwam to ensure longevity. Did this for a friend some 6 or 7 years ago and it looks wonderful. No sign of any support or Leylandii.
When I have removed such conifers ....did this last year.....I mixed as much compost, manure and,top soil. (from my veg garden) as possible Imto the site. At plantImg time I added generous quantities of fish look no bone. This summer the new plantIngs have been fantastic loving the comditions.