Naturally I'm offering a biased opionion, but the MX5 upright is a far better choice than the Sierra one. The main problem with the Sierra item is that it has been
adapted to fit, the offset mushrooms are there to try to give a bit more trail to the upright, this is what gives you self centering, where as the MX5 is designed for double A type suspension from the outset.
If I was to mix and match I'd use the MX5 running gear, with the later 6 speed box mated to a 2ltr Duratec. I'd also have a round tube chassis, but then I'm a tart.
TK