While researching for another project at the junkyard today. A 2nd Gen RX7 lower ball joint will bolt up to a BG lower control arm. The ball joint only fits the 3rd gen escort spindle unfortunatly, sorry 2nd gens. The ball joint end is 17.9mm for a 2nd gen RX7, 2nd gen Probe/MX6, and a 3rd gen escort. The 2nd gen escort is 15.9mm. The advantage to all this is the RX7 ball joint is longer by a 1/2" or 12.7mm (approx) there by increasing front track and negative camber, plus as an added bonus it is aluminum and lighter weight. I suppose the 2nd gen guys could drill out the hole to accept the bigger balljoint or get 3rd gen spindles.
All Factory Rx7 ball joints are Aluminum, if you go to a part store they are cheaper than OEM but they will be steal.
Oem ones are available for around 50-60 each at the site below