On long trips, your butt will go numb first due to the old style seats. But with the powerglide, you will have lots of stops for gas due to lack of higher gears. Even my 4 speed 350-2 could use that xtra gear.
These older cars are more set up for long trips of back roads, stopping to see the scenery, not cruising at 75 MPH on the open freeways,
If the latter, consider something with five (or six) gears.
You won't pull too much power increase without spending a LOT of money on engine upgrades. Simply throwing a 4 bl quadrajet with stock 4 bl intake will be a quick and (cheaper) way to (slightly) increase power. But the best thing is it will increase your mileage a bit unless you have a lead foot.
I also agree with Fuzz's comments about keeping her stock.
I used to be indecisive. Now I'm not sure. I feel like I am diagonally parked in a parallel universe. 1968 400 convertible (Scarlet) 1976 T/A - 455 LE (No Burt) 1976 T/A New baby, starting full restoration. 1968 350 - 4 speed 'vert - 400 clone (the Beast!) 1968 350 convertible - Wife's car now- 400 clone (Aleutian Blue) (Blue Angel) 2008 Durango - DD 2008 GXP - New one from NH is AWESOME! 2017 Durango Citadel - Modern is nice! HEMI is amazing! 1998 Silverado Z71 - Father-daughter project 1968 400 coupe - R/A clone (Blue Pearl) (sold) 1967 326 convertible - Sold 1980 T/A SE Bandit - Sold