I believe the oil return holes are in different positions. You can make the vertical head work good though,I built a GXV390 several years ago. You need to take some time on the head to get it right. The port is long and narrow and takes patience to open up and make the same size all the way thru. All the head parts are interchangeable, valves, springs, rockers, etc. even the rod and piston are the same, need to trim off the dipper on horz. rod. Only difference in cam is oil pump drive tang. There is a lot of parts being developed for the GX390 motors now as they are popular in kart racing. You should be able to build a stout motor for $1000. you can use the stock bottom end and focus your $ on head work. I have run a reworked oem con rod for many years, they work well up to the 7500 rpm range, just make sure your clearances are right and use good oil like fhs 62r, brad penn or the like. You can stroke the crank but really is not worth it for the result and the block can be easily bored and use the 390 piston. I would suggest talking to Tim O'Brien at EES , he helped me out alot years ago when I first started to use the Honda
Chuck Miller