Du Bro makes a nice 5 piece metric tap set that includes drill bits.
I’ve been doing some of the same. Except I’m going from 2 rail to 3. Assembling a milk train using 2 rail brass. I modified a Lionel truck much in the same manner your doing for a Butterdish car.
I found out the Lionel truck is a longer wheelbase than the prototype. The reason for the offset mount. It also ended up centering the truck better and avoiding some detail parts. I find the 2 rail wheelsets are hit or miss on 3 rail Gargraves. I’ll take the reliability every time if it’s feasible enough to do.