Removing 450WR cylinder

Just found a '72 450WR that's been hiding in a barn for twenty years. Surprisingly nice and original, but top end is stuck. Took off head and poured in a penetrating mixture, and then sat back and looked at distance between jug and lower frame tube--doesn't look like it'll come off with engine in the frame! Before I pull the engine, I just want to make sure there isn't some secret I haven't thought of.

The cylinder studs are what the head nuts are installed on so yes, you need to remove the engine. It is not a complicated task so go for it. You may even save the piston and rings as long as it it rusted and not a true seizure(lack of lube or air leak). Marvel Mystery works well if allowed to soak in for a week or so. You can also try a mixture of ATF and acetone. A friend at work and I were chatting about various rust desolvers and penetrants and he mentioned that was proven in a test to be the most effective rust penetrant over several commericial application including Liquid Wrench and PB Blaster

Plus one on ATF and acetone. Had the same issue on 390. Take pipe off and if you can get those rubber freeze plugs to block exhaust and intake(but see if you can fill from intake/exhaust side as we'll). Fill to max and I mean every where . Let sit for week then try moving stuff. Move stuff let soak , etc ...... I would also bolt head on and keep filling to brim. Keep checking cause some might bleed down past piston, which is good thing. If you get the cylinder up you can slide in some wooden door shims and tap piston down slightly with wood.

Thanks for the tips. I used my favorite mix of kerosene, ATF and Marvel Mystery oil. It broke free after soaking overnight, but there seems to be too much slop at bottom dead center, so the motor needs to come out anyway--at least it's easier to get everything loose while it's fastened to the frame instead of wrestling it on the bench. Sure looks like if they had just welded the coil bracket a few inches farther forward on the lower tube that you wouldn't have to yank the engine to just freshen the top end!