Siezed solid

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I am attempting to adjust the steering box on my vintage car but have struck a problem. The box itself is made from an alloy material and I need to undo an brass bolt threaded into the alloy, its stuck firm. Have tried an 18 inch extension but this has failed to loosen the bolt. Of course I am extremely anxious not to exert to much brute force for fear of breaking part of the steering box, which is virtually irreplacable. The bolt is large being about 1 1/2 inches across and the type to be used with an 'C' spanner.Its tempting to use an hammer and large blade chisel but again I cannot afford to be daft and break the box. Any tricks of the trade to help me out please?

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Cheers Rev-erand, so it looks like the hammer or blowtorch then

If I ask about alloy welding later you'll know why

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Thanks for interest and suggestions. The brass locknut threaded into the alloy steering box is the bolt thats siezed in, and through the centre of that is the brass adjuster bolt. Neither of which have been touched in decades I reckon. I have doused the area in penetrating fluid several times now and hope some may seep into the threaded area. This old car (1925)has certainly thrown up a few little challenges over the past two years or so of my ownership and I don't suppose for one minute its finished with me yet. :