sounds like a good plan. What's the actual question though?
20 inches is more then enough i'd say, id be dubious about 18 inches because of the slower burning H4350 powder but even that could be fine.
Cut it down to 20 and do a full test, then cut back to 19 and 18 while retesting.
Worst case is that the 7-08 doesnt perform to its full potential but you have 2 other rifles to do that with, and now you'll have a fast handling, lightweight rifle thats probably still going to be very accurate.
Do you buy 7-08 brass or just buy 308 and neck down?