Welcome to the forum JD, and as sad as it is to say, yes, a 361 will fit in the place of the 383RB (can't remember when it started) or the 413, or even rarer 392 Hemi. The 361 is a B engine, so the deck is lower, meaning although the bolt patterns are all the same for B and RB engines, the exhaust may be a tiny problem, but only if there is an H pipe, or pipe that connects the two exhaust pipes together, or if there is a single Y pipe, the exhaust manifolds at the dumps themselves is one inch narrower than the RB block and definitely different than the Hemi.
In short, yes, it will fit and bolt up easily, and what with gas mileage and gas prices these days, the 361 will only be identified by the few which really know the Mopar big block engines, and it will be an easy swap to change back to an original engine easy enough.
68RT you are right on that one, but the bolt pattern remained the same for the transmission for the "technically" big blocks, all the way down to the 350, 361, 383 B/RB, 413, 426 Hemi and Wedges, and 440. One thing I am not sure about is the bolt pattern for the 392 transmission, but do know it was different for the 354 and earlier transmissions, so, after doing a little research on the good old web, the 392 actually has a small block bolt pattern, but there are problems. Dowel pins are in the wrong place and the crank sticks out of the block rear so here is a link for an adapter from wilcap. http://www.wilcap.com/Hemi.html . Learn something new every day!