I think it's all in your head. I don't know what you weigh but lots of big guys ride SL frames without a lot of complaints. Are you afraid of breaking the frame? or just getting too much flex?
I don't really understand your comment about needing to spread the frame to put the rear wheel in? Is the 105 kit 8 or higher speed and the frame built for 6/8 speed wheels? I suspect the Bertoni would have the same issue unless someone cold set the frame for the wider 9spd wheelset. The Matrix Corsa Mondiale model came with Shimano 600EX SiS which I'm pretty sure was 6spd.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk