To a point …
I have an old 7.3 Ford diesel that serves as an everything vehicle: around town, hwy, heavy towing. I swapped the 3.73 to 4.56 when I went up to 35" tires. My mileage improved by about 15%. It also tows like a beast!
I also have an old TBI 350 / TH400 Burban with swapped 4.10s...
Both companies have treated me well, but I've ordered a lot more from Summit for over 25 years and they've never let me down.
I still check both sites when I need something, but Summit always seems to have the better deal or better parts or something and I know their customer service will be...
FWIW, I drive my turbo 400 backed motor all over the place at highway speeds and it's just fine.
The motor has to spin a little, but 70 mph is fine and probably about as fast as I should drive a 6-in lift 'Burban anyway.
I'm about to swap in a similar (slightly milder) 383. Other than a...