[Ackattack]

I have a 5.3L in my 68. Compared to the sbc I've had in these trucks (327, carbed 350, tpi 350, tbi 350) there is no doubt that the 5.3L is way better.

My truck has 4.10 gears and the 4L60E and doing 75 I'm getting about 16.5. However, I had a '05 CCSB Z71 with 3.42 gears (and 32" tires) and I would get 20+ mpg out of that in WA where the speed limit was mostly 60.

Also, with the 5.3L, you can bore it out to a 5.7L and use the stock LS1 pistons in it (same stroke as the LS1).

IMO if you don't have an engine or your old sbc engine needs a rebuild, you'll be happier with the 5.3L.