The length of the 8-710 would be less - 4 cylinders long vs. 4 but the bore spacing would be the same as the pistons are the same diameter. I think the height of the crankshafts are the same as they both mate up to the same generator on the ECCO conversions. The 710 is essentially a 645 with a 1 inch longer stroke. The piston rings are the same as are the cylinder heads.
I think that it was hoped that they could achieve a design that could use a lot of 645 parts and I think that they were in a hurray to get a higher horsepower engine without too much design work. From the end view the 710 would be slightly taller above the crankshaft centerline.