Here is the complete 'wrestling' verse that was quoted partially at the beginning of the former post:
"For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places" (KJV) (if you're wondering about 'principalities' and 'powers', look at any one of Walter Wink's books; he elaborates the two words thoroughly.)
In the Preface to Milton (a hymn which came to be called Jerusalem) he undoubtedly had Ephesians 6:13-15 in mind. Here are the salient passages:
"Bring me my Bow of burning gold:
Bring me my Arrows of desire:
Bring me my Spear: O clouds unfold!
Bring me my Chariot of fire!
I will not cease from Mental Fight,
Nor shall my Sword sleep in my hand:
Till we have built Jerusalem,
In Englands green & pleasant Land.
The harvest (the plow, the sickle, the winepress) are vivid images in Blake and in the Bible, usually meaning much the same thing:
From Revelation 14:
 And another angel came out of the temple, crying with a loud voice to him that sat on the cloud, Thrust in thy sickle, and reap: for the time is come for thee to reap; for the harvest of the earth is ripe.
 And he that sat on the cloud thrust in his sickle on the earth; and the earth was reaped.
 And another angel came out of the temple which is in heaven, he also having a sharp sickle.
 And another angel came out from the altar, which had power over fire; and cried with a loud cry to him that had the sharp sickle, saying, Thrust in thy sharp sickle, and gather the clusters of the vine of the earth; for her grapes are fully ripe.
 And the angel thrust in his sickle into the earth, and gathered the vine of the earth, and cast it into the great winepress of the wrath of God.
 And the winepress was trodden without the city, and blood came out of the winepress, even unto the horse bridles, by the space of a thousand and six hundred furlongs.
Now look at Milton, Plate 25 :
"Loud shout the Sons of Luvah, at the Wine-presses as Los
With Rintrah & Palamabron in his fires of resistless fury.
The Wine-press on the Rhine groans loud, but all its central
Act more terrific in the central Cities of the Nations
Where Human Thought is crushd beneath the iron hand of Power.
There Los puts all into the Press, the Opressor & the Opressed
Together, ripe for the Harvest & Vintage & ready for the Loom.
They sang at the Vintage. This is the Last Vintage! & Seed
Shall no more be sown upon Earth, till all the Vintage is over
And all gatherd in, till the Plow has passd over the Nations
And the Harrow & heavy thundering Roller upon the mountains
And loud the Souls howl round the Porches of Golgonooza
Crying O God deliver us to the Heavens or to the Earths,
That we may preach righteousness & punish the sinner with death
But Los refused, till all the Vintage of Earth was gatherd in."
Now focus on the valley:
Joel 3:12-15 (King James Version)
 Let the heathen be wakened, and come up to the valley of Jehoshaphat: for there will I sit to judge all the heathen round about.
 Put ye in the sickle, for the harvest is ripe: come, get you down; for the press is full, the fats overflow; for their wickedness is great.
 Multitudes, multitudes in the valley of decision: for the day of the LORD is near in the valley of decision.
 The sun and the moon shall be darkened, and the stars shall withdraw their shining.
And this from Blake:
14Multitudes, multitudes in the valley of decision: for the day of the LORD is near in the valley of decision.
Of Vision, they scent the odor of War in the Valley of Vision.
All Love is lost! terror succeeds & Hatred instead of Love"
(Jerusalem 22:9 Erdman 267)
Here is a web site on the Valley of Decision.