Neither are going to be "fast" or "quick", considering their 4500lb + weight. Get one with the 2.71 rear axle ratio. I would suspect the 400 would get better highway mileage, but that could be a wash.
In 1974, in the "heat" of a "fuel crisis", Fenner Tubbs C-P took one of each of their demo fleet and did their own mpg test. They used the Shell station across the street from the dealership as the fueling point. Each car was filled with fuel and then drove south to the Lubbock city limits and then south to the edge of the Caprock. Then turned around and headed north back to the Shell station and the dealership. The fill points were standardized and observed. Then everybody returned to the dealership and the paperwork was done. The highway speeds were 55mph using the factory cruise controls. This was during the national 55nmph speed limit.
Results -- Plymouth Gran Fury 360 2-bbl, 19.66 / Chrysler Newport 400 4-bbl, 20.33 / Chrysler New Yorker 440-4bbl 20.66 mpg For verification, these results were notarized by the dealership's Notary Public. They printed the results, with a sheet for each car, on company letterhead, notarized, and placed prominently on a table in the showroom.
Obviously, many GM owners scoffed at these results, using their Oldsmobiles and Buicks for comparison. If one of these owners was interested in a new Chrysler, they got the keys for a weekend. They all came back and bought Chryslers on Monday.
Now, during this time, I was closely monitoring the highway mileage on our '66 Newport 383-2bbl (Holley 2210 at that time). I was using the amount of throttle percentage to gauge load and ultimate fuel use (air/fuel ratio). My trusty K&E sliderule assisted. I was making it as scientific as possible. On my trips home, I was keeping a log of time/miles spent in each operational mode (city/highway). On the highway portions, the old Chrysler (over 100k miles at the time), using my calibrated foot in place of a cruise control, got just over 20mpg at 55mph. For a car that liked 75-90mph cruising, it was boring "hell", but we got through it. The fantastic long-range reception of the factory AM radio helped. On that first trip, I saw a recent Corvette abiding by the new 55mph speed limit and KNEW what he was experiencing. The mpg and the 55mph Corvette made me smile. Remember, too, this was in flat west Texas and started on the same highway the dealership used for their mpg testing. Before the speed limit changed, the Chrysler would normally average 15-16mph for the same trip, depending on how much time was spent about 75mph. With the 2.76 axle ratio and H78-14 tires, 55mph was 1900rpm and 75mph was 2600rpm. On the new Chryslers, with their larger 15" tires, the rpms would be less.
If you are looking for a car, go into
www.hamtramck-historical.com and the Order Guide for the potential model year and look for the axle ratios chart. Seems like they were juggling 2.71 and 3.21 axle ratios aorund a bit back then. Look for the respective option codes so you can look at the Data Plate for verification. Personally, I'd aim for a 2.71 ratio and NOT worry about how much rubber it might lay, but lower rpm at highway cruise mphs.
As for modifications, just a more modern carburetor and a matching ignition system would be the main concerns. As archaic as the OEM 4bbl intake might appear to be compared to an aftermarket Edelbrock unit, I have been surprised how a 1958-era Chrysler 2x4bbl intake made over 500lbs/ft or torque on Nick's dyno one night. That shouldn't have happened, but it did. For general use, though, the Edelbrock might look and be a bit better, though. Perhaps a Street Demon 650 rather than an emissions-compromised TQuad? Or an AVS2 with annular venturis? Several good options that hide under the factory air cleaner.
Just my experiences and observations. Your experiences might vary.
CBODY67
