Both N270BC (former HB
-IGC) and N705BC (former HB
-IGD) were fitted with the higher rated PW
JT9D-7R4 engine which is very rare to find on a 747-200. There are only a handful of 742s with this engine, but they include NW
's passenger ships 6631, 6632, 6636, 6637 and 6638 as well as cargo aircraft 6744, 6745 and 6746. All the other NW
742s are fitted with the earlier variant JT9D-7Qs which are much more common. Hence, acquiring those two airframes just for the engines and spares was a financially attractive proposition.
Additionally, both aircraft were already fitted with side cargo doors (since they were delivered to Swissair as Combi variants), which would have made the conversion to freighter an essentially painless process if NWA so desired. Alas, the aircraft were mothballed in late 1999 so don't expect them to ever fly again.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada