for '87-9 ALL M/T (non-L body) TD
's used the 520, N/A & Turbo.
except the TII/IV cars which used the heavier-duty 555.
so it doesnt HAVE TO be from a shadow...
however there were two FDR's available.
(similarly the axles can be from any 87+ EEK (not van or L-body),
not just a (P-body) shadow)
if you found a 555 that would be interchangeable with yur 520.
in 90 the 520 was upgraded to the 523, also interchangeable,
so if you found a 523 that would work too..
that also has different FDRs available.
however, that is more work since you need to swap out the shifter and cables as well.
see DDG- The Transmission Guide