You also may want to go up on the rear end ratio.
308s are great for highway as long as you stay away from the hills. Had 333s in my 39 and had to downshift to get up any kind of hill. Might also explain some of your bog. 355s are a good bet for all around, decent gas mileage and better out of the hole. Used these in the '39 went down 2 tire sizes from an L to about an H, I think that is about 235 R 15 and picked up about 3 miles to the gallon, as the cam liked it alot better. Cams have a sweet spot where they perform well and get the best mileage. Carbueration, cam, gearing is a complete package and many over carbuerate. Had a friend in MN that was running a 427 - 67 Chevy with a 450 2 barrel and it was fast, like in the 12 second bracket. Hope this helps.
