Lemme put it this way....
The 1k (which seems lofty to me, but i dont know your location), youre thinking of dishing out for a junkyard 460, that you know nothing about, that may or may not be in good shape...would buy you all the mods i mentioned above, PLUS a 400 rotateing assembly to put in your 351M. Then youll have an engine you KNOW is reliable, and will perform just as good, if not better then a healthy, stock, 460.
The 390 i wouldnt even consider at this point.
Will the 400 make 350-400hp with what i suggested? No. But itll make gobs of low end, reliable grunt and youll have 1/2 (or less) into what you would a 460.
The 460 cheerleaders dont like hearing this, but compareing engines from the same era the 400 makes 80% of the 460s peak torque, and at less RPM, mostly due to its 4in stroke. So its not rocket science to conclude that with mods, you can surpass a 460 for torque.
Lemme tell you this...
I towed a 7000lb 32 ft travel trailer around for 2 years with my 77 F250. Thats a bone stock 400, saddled with a C6 and a 3.07 rear. Granted, it wasnt ideal keeping 55 up a decent hill, but it always pulled very well (considering the gearing) down low. To me, that speaks volumes as to how much grunt these things make.
I know yours is a 351M, but like i said, a 400 is just a crank and rods away.