I've just used bits of the Heritage Mk IX conversion on my Trumpeter Spitfire Mk VI, converting it to a Seafire III. I was very disappointed with it - designed down to a price rather than up to a standard - lots of stuff missing that would be needed make a Mk IX and no instructions on what more is required.
My exhausts were short shot too but just about useable - I was surprised that no one offered the six stub exhausts as a seperate add on - it opens up both the Airfix and Trumpeter kits to late model versions of MkV etc.
I used the exhausts, the Aero Vee carb intake (which required re profiling) and that was was about all, the extended cowling was warped, the tailplanes would not fit the Trumpeter kit (no surprise) and the prop was worse than the kit one in detail.
There were no parts to effect the wing mods, no late cannon barrels, no later windscreen etc etc. Sorry to sound harsh but it could have been so much better and I would happily have paid more - after all the Spit is not that big in 1/24 that you couldn't have 2 or 3 in the collection so I wouldn't have thought it wouldn't be a one off purchase for a significant number of modellers.
Edgar, I didn't know the Airfix MkVb kit had the later type windscreen, it's the weak part of my conversion - I must investigate.......