I have a 1903 Mannlicher and of cours scope mounts are a little hard to find and expensive. I was looking around and ran into this <http://www.brownells.com/userdocs/products/l_962070070_1.jpg> and was wondering if it could be modified to work. Surely one could get close, I mean they mount those hideous side mounts on 94 Winchesters and they are 2 or more inches left of the boreline. Thanks in advance.

Uhgg! Don't do it.

If you must mount a scope you can find the proper two piece mounts.

I once had a 1903 drilled and tapped and installed a scope using the correct mounts. Nevertheless, I regretted modifying that rifle for a scope. It was so light and handy before and afterward the scope made it feel and look all akilter.

I wouldn't drill an original 03 Mannlicher.

If you want the proper side mount then I can help, I have a brand-new complete Jaeger single-lever one that's never been on a rifle yet. If you absolutely HAVE to D&T that 1903 then IMO a good side mount is the way to go. Lots less awkward and the iron sights are still usable if wanted.
Regards, Joe