I am running a thick wall 1679cc SP right now. I love it, and for its size, its a blast. I run the living snot out of it daily. I am running a center mount, and I can't make it get less than 20mpg. If I am really nice to it I can get close to 30.
Here are my specs;
Old H case, full flowed
040 single port heads, lots of porting like in the How to Hot rod book, 3 angle,
8.7cr, .046 deck
AA 88mm, slip in at the case, 90.5 cut at the heads,
Stock crank and rods, balanced
10lbs flywheel
Center mount 40mm DCNF, 32mm vents
Old Mallory dist.
non-doghouse cooler, no extra cooler.
CB Bigfoot 2208 cam, w/CB 30mm lifters and dual springs.
1 3/8" header and hideout
What I am going to change:
-Get rid of the cam. I knew it would be loud and hard on parts, but I wanted to experiment. It pulls amazing, and makes a ton of torque, but its really noisy. I swapped in aluminum push-rods, and that only helped a little. I think I will try a boring w100 next.
-Use Kads instead. The center mount does work really well after a bunch of work to the header and a hot air box for the winter, but Kads will most likely work better and be way easier.
Hope this helps.