There really is no ”Best year”, it’s really gonna come down to best find, or best deal. Ideally, and 89 to 92 will be the most direct fit since the chassis they come in are the most similar to the RC. Additionally, a 4wd donor will also come with the most correct front axle and a matching right drop transfer case. The main downside with a donor in those model years are possible lack of an intercooler, 727 transmission, VE injector pump H1C turbo And Dana 61 front axle. Keep in mind that those components can be replaced or modified but it will add cost that may not exist with other model years. As you get into later model trucks, you get more powerful and more desirable versions of this engine (2nd gen engine in particular) and better transmissions, but you can’t take advantage of their transfer case or front axle because Chrysler made big changes there that make them incompatible with the older trucks. 3rd gen Cummins engines began to use computer controlled VP44 injector pumps. It’s simple enough to still be a good candidate for a swap but that particular ip was not very reliable. After that, you start getting into computer controlled engines but this is also the start of the 24 engines. All engines share the same architecture and therefore can be mixed and matched in almost unlimited ways. Want a first gen engine with a 24 valve head, it’s possible. Want to decomputerize a 6.7? It can be done. So, in general anything is possible if you have the bank account.
You brought up transmissions. It’s possible to install almost any big truck transmission, including big rig transmission. An Allison behind a Cummins Isn’t out of the ordinary. It’s done a lot in commercial trucks, and speaking of which, your donor doesn’t have to be a Dodge Truck. Many commercial trucks, school buses and construction equipment came with these engines. Some will require more adapting to work, but the Cummins were designed to be universal. There also 3 cylinder versions and 4 cylinder versions that you might consider too. In fact there is a propane version which uses spark plugs and a distributor.
One last thing, there will be a potential added cost in terms of supporting mods. The 6BT is heavy and torque output exceeds every other engine you could find in a RC/W150. Highly recommended is a one ton front and rear axles
it all depends on what you want, what you can find and what you can pay