Does the 8.375 oz include battery, ESC, motor etc?
If so, you could be fine at 0.9/1 thrust to weight (although more power or less weight is always good) but a 50% viggen has a wing area of approx 80 sqin or a little over 0.5 sq ft. This would put your wing loading at 17 oz/sqft and your cubic wing loading at ~23, so you'll need to launch her hard and keep flying pretty fast.
Worth a try I would say.
DamoRC