Ray, it's actually not very hard, just use CA to fill the gap, a big file to begin with, then normal sanding, I have one of those Small battery powered sanders that Runway 13 used to sell.
takes me about an hour total, the engines are pretty good by themselves, attaching a resin engine may be marginally quicker though, hardest bit with that is getting the part pylon off the fuselage.
I know this from currently trying to build the Red Roo MD95 (Boeing 717 if you must), but another significant problem has stalled that one.