Hey Cairnsy;
I worked on and crewed CH-47As, Cs, and Ds, and worked for Columbia Helicopters on Boeing 234s. Ish47guy is correct.
CH-47As were the only airframes that had the tapered aft pylons. They also were the only ones to carry dual wheels on the aft landig gear (this was replaced with a pair of single wheeled aft landng gear)
He is also correct in stating that the 'squared' off aft pylon trailing edge that was introduced on the B-Models, was done to improve directional stability.
Other features added to improve stability include the ventral strakes on the aft lower section of the fuel pods and on each side of the ramp, and the offset strakes on the forward part of the forward pylon.
The wider chord FRBs with accompanying rotor heads were retro fitted to C models in the mid 1980s and were standard equipment on D models. As far as engines go, you could probably get away with installing the smaller intake screen as used on A and B models and on some C models, but I believe by the time the wider FRBs were introduced all the C-models had the larger engines and larger intake screens (The engine combinations start to become convulted after about 1981 or 82... The unit I was assigned to in Korea in the early 80s had aircraft equipped with at least two seperate versions of the T-55-L-11s, while our sister unit had some aircraft equipped with the less powerful T-55-L-7s.
The last time I was in Korea the aircraft we had were all D models and were equipped with T-55-L-712s..But there was no outward apparent differences between the L-11 and L-712 series. They both had the larger intake screens and the exhaust extended the same length from the engine cowling.
Here is a link that might be useful.
http://www.aviastar.org/helicopters_eng/bvertol_chinook.php
HTH
Jonathan Primm