I did that exact swap. I bought a rolling chassis with what I thought was most everything. The frame is different as mentioned in that it has bolt in, heavier crossmembers so that the trans can be removed from under the truck. The brake line routing is different for the same reason. I was told that the master cylinders were different but they appeared the same although I did not risk it and used the H-M master cylinder. Make sure you have all linkage, the sterring column is actually the same except for the shifter, the indicator will bolt on where the 3 speed shifter mounted. There is also a neutral start switcj mount that clamps on the column under the hood. Starter is different because it uses a solenoid start. Driveshaft length is different. There will be a pushbutton starter swith above the headlight switch. The transmission cover plate is different to allow access to the bellhousing bolts. Although there were different cabs for this cover, your 3 speed cab can be cut to fit and will be close but without the recess for the cover and actually, there are some strengthening ribs that are slightly different. H-M used a different carb with an electric choke but that is ot really an issue. There is an adapter plate that goes between the engine and transmission. GMC and Chev use completely different linkage and shifters so your donor vehicle needs to be correct manufacturer. The pivot on the side of the block is different and in the other locating point. All this being said, I really enjoy the truck more now that it is H-M and would not consider having it any other way now. Here are a few of my webshots pics that show the switch process. If you look around my webshots, you will also find a display pic of everything needed for the swap on a GMC. Enjoy!
http://rides.webshots.com/album/399976892eMbHYm?start=0
