Purely what the previous owner said. Has had a lot of work though with all receipts. Has different internals and carbs, exhaust etc to name a few.
Not fast though by today's standards but it's not about that I suppose.
Don't get me wrong bud, it looks great and sounds in top shape mechanically.
The 289 needs about £30k put into it to get it up to 400bhp. The standard ones were 190ish bhp when they left the factory. The high power ones (big money for an original hiPo block) were 270ish. There is a 400bhp FIA mustang notchback up for sale, but it's £60k, so unless your sister paid that much then the old owner is being generous with the truth.
This 289 has been in my garage for the last 7 months, having a complete rebuild, pistons, solid lifters, cams, ported and polished head, updated fuelling etc etc. it already has flow master exhaust on it and won't be making anywhere near 400bhp.
Ignoring the power claims, it looks brilliant and I hope she enjoys it. As you said, these aren't all about modern day "speed", although in a straight line they are quite punchy still. It's the noise, occasion and "cool" factor that make these epic. I'd love one, irrelevant of the power output.