With a 243 and a 100 I'd run 35-37 grains I4064... It's always been a decent load for us.
As to Ruger #1 rifles... I have a #1 standard in 338win mag. She's been a heck of a rifle over the years and I have hundreds of rounds through her. My brother has a #1 in 300 H&H.. It's a good rifle too.
The #1 is actually a rather accurate rifle, but you have to be dang careful on your sandbag pressure and bagging. I fired a cloverleaf at 100 the other day, then slid the sandbag out away from the mounting screw (where your bag ought to be centered) and the same load/rifle shot over 2"... As far as bag pressure, just a bit of down-pressure to settle the rear is what seems to work best.