I'd not thought about this, but Ross is correct; the scheme he
suggests ought to work quite well. Guide 8 covers (between the
two disks) elements back to JD 2437000.5 (February 1960); you
could copy over Guide 7 elements covering dates between then
and JD 2439800.5 (June 1940).

I expect that my first step in dealing with this problem will be
to write a program that can read in an MPCORB file and do the
numerical integration to any desired date (said date limited only
by your patience; it may take a while to integrate 150,000+
orbits over a few centuries... fortunately, for most uses, you
would only need integrate over time spans of perhaps a few
months, to bring MPCORB to a "current-day" epoch.)

-- Bill