Ponz, I have a similar problem. In my case, I am limited to ~O54. Unfortunately, most of the large steamers require O72 minimum. Have you considered RailKing Imperial? The RailKing Imperial does not have the detail of Premier and are "semi"-scale at best but they are still bigger than traditional non-scale offerings. Take a look at the MTH 30-1812-1 PRR M-1a as an example. I just picked up an Imperial PRR S-2 (MTH 30-1785-1). It's not nearly as nice as it's scale counterparts, but I can run it on my layout and IMHO it doesn't look out of place pulling larger rolling stock.
Tony
