The 3:36 rear end is the factory ratio used in a 56 car with an auto trans. Since the 350 trans is still 1:1 in 3rd gear the final drive ratio wouldn't be any different than using the stock PG trans. In my opinion the 250 engine would be fine with the 350 trans and the 3:36 rear end for your stated driving needs.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine