Yup, acceleration above 70mph is slow coming on, but my '99 will push 85mph with my 250+ pounds. Yours should get there too if set up right. I haven't checked the jetting, but it has a HD Dyna muffler with modified baffles, and it also has the biggest Dunlop 404 that will fit on the back, so the gearing is tall. When my speedometer gets barely over 80, I figure I'm probably going 85+. Sure putts along nice at 65
I don't think I'll go with a chain conversion on the '87 until I try the big rear tire on it too. I've already got a Cheng laying around here that is a tiny bit bigger than the Dunlop. Hoping that may solve all of its high revving on the highway, but it is gonna make first gear kind of tall on the 4-speed.
Your "bad cut out" at top end is a sure sign of something amiss. Read your spark plug. Jet accordingly. Hope nobody burned anything up running it too lean.