By the time you pay for the press work and alignment, it's about a push cost-wise to rebuild your 400 crank with a new 426 rod kit or buy a 426 crank from the TT store. However, the earlier cranks used a woodruff key to keep the primary drive
gear from spinning on the crankshaft, and the 426's (at least from 01 up) use a splined shaft, which means that you would need to buy a primary drive gear as well. Depending on how worn your gears are, it may also be a good idea to replace the primary driven gear (that's riveted to the clutch basket) as well. If it were me, I'd go for the new crank and primary drive gear from the TT store, and see how well it meshes with your old driven gear...it will prpbably be fine, and you get the piece of mind of having new parts.