I wouldn't force it either. If you can't get it loose, pull the head and work directly with the cylinders. With the head off I'd shoot some down the holes where the push rods come out and get some on the cam...maybe will migrate to the cam bearings.
One would assume the lower end (crankshaft bearings) are ok as they were oil soaked but one never knows. I'd drop the pan and inspect mains and rod caps to ensure they are lubed and while the cap is off, I'd put some Lubriplate 105 {engine rebuilding grease) in the white tube, turquoise cap at any auto parts store on the bearing inserts before replacing the caps. On that note, I would snug them up and after you get it broken come back and torque to spec or replace if worn down to the copper.
One other thought is while you have the rod caps off for inspection, after inspecting, leave 3 of them off and try to break it with only one cap reatached...connecting that rod to the crankshaft...... the piston at TDC or nearest to it. Once that one breaks, just enough to get it moving. more PO to it and then leave it be, cap a second one and break it loose and so on.
On the cylinders with the piston at the lower portions of the cylinders, maybe take some fine sand paper and PO and take the rust film off the cylinder wall before adding that cylinder to the ones already broken free. Once you get them all broken free, I'd get out the hone and do a light honing with that piston at the bottom....then flush out everything and in doing that, move that piston to TDC making it easier to get all the "finds" out of the cylinder.
Also inspect the cylinder walls and valves for condition whereby you may be ok to put it all back together or will need to do an OH on all or at least the head??????
You are going to to a lot of time and trouble here so you have to ask yourself is it worth your time. If so, stay with it. I have done an inframe without having to pull the engine but that was on an engine that had upper problems....aluminim piston related primarily, with the cylinder walls and crankshaft journals in usable condition. The head did go to the machine shop for typical head work. That tractor is in use today, some 20 years later and still works like a charm. If you have to pull the engine it means a split and then you might as well do the clutch, pressure plate, starting ring gear (probably worn), throwout bearing and all that too.
One is assuming that you have the tranny in N. But to be on the safe sice, have somebody push in the clutch while trying to break it.
On penetrating oil, when in industry, the mechanical guys used Kroil brand penetrating oil. Comes in a pint can or 16 oz aerosol. Pricy, but a high quality penetrating oil that works....just ping the www.