Some comments:
1) I made 3 trips (in 3 different years) from my home in Alberta to Bowling Green, KY to the GSCA Nationals with my 68 GS400. I would also tack on a vacation (Orlando one year, Corpus Christie the next, etc.) so our mileages would be very similar. These trips were made on fresh oil and a fresh filter of course but no oil change midway. My car was geared 3.64:1... much lower than yours (ergo more engine revolutions per trip) assuming you have factory gears in your Electra. This was done in the early 80's on SF Castrol GTX 20w50 oil and an AC Delco PF24. Probably 2 quarts of make up on the trip as I recall. No oil / sludge issues ever. With the better formulas today I'd have less concern now even. ZDDP, or lack thereof in modern formulas, is not an issue with the spring pressures you have in your car (assuming stock of course) and Buicks do not wear cams anyway. They have oil pump and front cam bearing issues due to the design but the cam / lifters are not an issue. ZDDP is a moot point if you are using the BP oil anyway.
2) Wix still makes the longer filter if you want, I couldn't run it on my GS because it was too close to the sway bar, if you have room in the larger Electra put the long style BOP (Buick, Olds, Pontiac) filter on.
3) Not as much mileage and a different engine but still an old carb style, I took my 1970 GTO on its last trip to Portland actually in 2011 (went on many road trips in that car too, long ones), that was using modern Shell Formula 10w40 SM and a PF24. No oil change midway and no issues.