I think that the oil cooler mod(to thermostatic) is almost a necessary mod, since the factory system only opens the cooler circuit above a specific pressure(~40psi).
I'm pretty sure that the oil needs to be cooled on occasions when you are at less than that psi. Other oil mods include shimming the oil pump, to improve pressure slightly,
lowering the oil pickup to reduce the chances of oil starvation, running an extra quart of oil(for the same purpose). There is also a high flow oil line supply mod, to replace the tube
and banjo fittings coming off the oil pump itself. I agree with the OP about replacing the rad with an aluminum one- it alone cured my high temp problems. Not because
the factory unit is insufficient, but mine was simply old and clogged, and the AL unit has a larger capacity.
I also agree with the 87MGTE about necessary fuel and air mods. Some may not be necessary to hit the 400 mark, but without them, you're running on the edge of the safety margin.
Also- a full exhaust is necessary- and should be one of your first mods. I would replace the DP and exhaust, as well as intake first, and see what a difference it can make in your car- it's pretty good.