Not really.
Not only is the M77-RSI/mannlicher barrel shorter & of a thinner contour (same as the UltraLightweight), the RSI stock is measurably thinner - thin enough that a widening of the barrel channel is conraindicated.
The only Ruger Model 77 suitable for the conversion would be the M77-UL (UltraLightweight), due to it's suitable bbl contour.
That advice, however, is for using a RUGER RSI stock - a regular Ruger M77R or M77RS can be mannlicher-stocked via purchasing the stock from one of the aftermarket stock making companies, like great American Gunstocks, etc, etc.
Of course, it won't be a drop-in, and will also be uncheckered/unfinished.
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