Yes the 240 head has smaller chambers. I have a set of 390 pistons I will use and the compression will come out around 9 to one depending on how much the machine shop takes off.
I have owned a few and one I rebuild back about 92. I had a 260 cam and did some head cleaning. It ran like stink, even though my intake and headers never came from PAW and I ended up with the stock stuff back on it.
So far, this is my plan, elcheapo. I have the new Badger 390 4.050 pistons and rings which will make it around 308 I think. I am looking for a 3/8 stud mount head so I can run Chevy 6 rockers. That is a cheap trick good for 5 horse. I will be at 9 to one or a bit higher, have a ported head, efi split exhaust and maybe an offy dual port intake with a 390 holley, or a bit smaller 2100 autolite 2 bbl.
I gues the stock cam is about the best for low enf and milage so I will stay with that.
That's my plan anyway.
Then put a ZF 5 speed behind it and find a truck to put it in, maybe the 69 crewcab.