The winglets normally fitted to the -300 are shorter than those on the -800, the -800 are quoted as being 8ft 3 in, don't know the height of the ones on the -300.
until the arrival of the Zvezda kits there was no decent -800 winglets anyway, there are the ones in the Daco -300 kit, but they are actually taller than the ones in the Zvezda -800 kit
so not correct, I did have at one time a set of drawings showing the difference, but seem to have been lost in an old computer failures, or they may be held hostage by the thieving scum at Photobucket.
maybe someone else may be able to help.
as to where to cut the wing it depends on the winglets you choose and what kit you are using, but just inside of the navigation lights and where the wingtip and your chosen winglets match in span.
If you have a set from a Revell -800 laying around they are too short for a -800, but may be close to suitable in height, just need to trim off a lot from the front to make the tip shorter in span.
this gives a comparison for the -800, so shorter than that.