SuPer SiX Slant Signet!

I vote stay with the 170/, it is a shorter stroke and good low end power, and I have had no problems at all with a 390 Holley and it runs smooth and has been for about two years.
My 66 is light, 2,700lb's and I run 3.23 gear open 7 1/4 rear end with 3 on the tree standard shift. open the exhaust to 2 1/4" pipe with a muffler and 14'' 215/70 tires.
I am positive I am getting at lest 24 mpg... wide tires, under inflated tires and drag on bad axle and wheel bearings will make your mpg go way down.
Put some kind of a fresh air system on it and it will help !! It did mine, It's 200 + degrees under the hood and that will bring your mpg down. I say go with the Low deck 170/.
It has worked for me and is my daily driver :glasses8:
I have removed the temporary rubber gas line and bent a stainless steel line, the breather is from a 93 Jeep 4.0 and the battery is in the trunk.
Just an option to think about and I wish you well :colors: