I just finished building the one in my profile picture.... Biggest issue is fitting a front end to that massive neck. I have put two different front ends on it. The springer in the picture, and it now has a triple clamp mountain bike front end on it... It is getting turned into a "dirt bike" with a 48 tooth rear sprocket from the 4 stroke kit..
The way I fit the front end was easy to do, I used a dremel tool with a orange grinding stone, mounted the black cups into a vise, and went round & round the inside until the bearing cap I wanted to use fit the hole, then used the monark bearing cups, pushed down into the black stock ones used as a bushing... I had to re do it again to fit the 1 1/4 mountainbike front end, so yes, they will sand out that big... you'll probably burn through a couple of stones, just make sure to try to keep the opening as round as possible..