KT88s are usually coke-bottle shapped rather than the straight glass of most 6550s but it depends on the manufacturer. An SVT should have plenty of room.
6550s come in a variety of shapes:
Most KT88s I've seen are either shapped like the tung sol 6550 in the pic above or just slightly bumped out like the genalex.
Or there are some like EHX which look (nearly) identical:
