Read about the differences in 1970-1972 and the 1973 and later power steering sectors
HERE. You did not say if it was a power steering gearbox but, basically, the 1973 and later power steering used the large-sector box while the 1970-1972 used the small-sector box.
The manual steering boxes used the small-sector pitman arm for all E-Body model years 1970-1974.
It sounds like you have a small-sector box.
There are
small-sector pitman arms on Amazon. The MOOG
K7102 is the small-sector pitman arm and is getting harder to find.
Check the casting number on the sector. These are the early E-Body (in bold) casting numbers which are small-sector boxes.
1970 Parts Catalog:
2537397 = 1970 A, B & E-bodies (non-T/A box)
2948900 = 1970 E-bodies w/quick ratio T/A box
1970-'71 Parts Catalog:
2948991 = 1971 A, B & E-bodies (non-T/A box)
2948900 = 1971 E-bodies w/quick ratio T/A box