1973 CB750K3 Transmission Repair

[Hybrid_Phoenix]

Hi all, I'm currently attempting to tear apart the tranny on my cb750 because 1st gear is basically screwed and needs replacing.  The problem is that I can't get the screws on the top of the engine block to come off, they're frozen and/or stripped.  Is it possible to work on the tranny from the bottom with the engine block still attached?  Or do I need to take it into a shop to have it torn apart?

Thanks.