B/RB block usage in '65 C-body cars was pretty cut and dried. 383 2bbl, 383 4bbl, but don't recall the RB426 being in the mix (usually B-body only), but 413s were in other C-body cars (440 came in 66). I remember a road test in Motor Trend of a '65 Monaco 500 2-dr hardtop with a 413. I think the 361 would have been in the B-body cars, as it was an option in the later Dodge Charger a year or so later.
Check the rear seat cushions for build sheets stuffed between the springs and burlap backing. Rear axle ratio, generally on a tag on one of the cover bolts, would have the ratio stamped on it. A small tag. 2.76 standard on 383 2bbls, option of 4bbls. 3.23 standard on 4bbls with 2.76 optional . . . per general Chrysler practice back then.
Generally, I'd suspect a 383 of some sort. 2bbl had single exhaust on the rh side of the car. 318s had their single exhaust on the lh side of the car, for reference.
The 413s and 426 Street Wedge did not have the performance increase we might normally believe they should have had over a 383 4bbl. A MAIN reason was the exhaust manifolds had 2" exit holes! Later 440s had larger exit hole diameters, like 2.25" diameter, plus a better design and cross-section. A set of '68 HP manifolds would be much better. 413s were 340 horsepower and 360 horsepower, with the 426 Street Wedge being 365 horsepower. Not a lot of difference or real advantage over the 383-4 325 horsepower, on paper or on the road.
Main difference in the transmissions would be the specific governor item. Probably no real internal differences, as to clutch pack number of plates and such. Torque converter might be close to the same, too. There's probably a parts book you can fine online to download, if desired. There's one website I found that specializes in Chrysler 300s and they have parts books for download, up to about 1965 and back to the later 1950s.
That's all I can think of right now.
CBODY67
