They may be a bit tight on the clam clamp. I tried some JJKT88s that sounded great but it snapped my clam clamp - but the base was flanged much more than the GT was.
At least in VH4 and in all other amps I have fitted KT88 or 6550 tubes, you need to remove the clamps. Two bolts per retainer = approximately less than 5 min altogether. 10 min time and lots of cursing to get them back.