Michael,
I just completed this very project. The seals around the three shift
rods are felt washers, not seals per se. These were shown in the XK’s
Unlimited catalog and my wife purchased three as part of an XK Christmas
present. Upon dissassembly, I found that the XKU washers were much
smaller than the original felts. By smaller, I mean that they had less
“meat” to them. After due consideration, I loaded the original seals
with grease and put them back in. My car is at least a few weeks from
the road so I haven’t been able to challenge my fix but I would guess
that the grease will work pretty well since it won’t be readily
dissolved by the tranny oil. I used regular wheel bearing grease but,
if I had it to do over, I would probably use Silicone Hi temp disk brake
caliper pin grease. This stuff is almost impervious to oils and will
not dry out over time. Come to think of it, since I have to remove the
shifter “buckets” to install the overdrive, I will get a chance to do it
again 
Brandon,
A 67 E box should be all-synchro. My fhc had one installed at one
time. The person who installed it said that it was the best
“improvement” he ever made to the car (originally a non-OD car). I
think you need the E bellhousing, clutch linkage, slave cylinder, and
U-jont yoke. The E-type shift lever is farther forward than the
standard XK tranny so you will have to cut a hole in your tranny tunnel
and buy new tunnel carpet. I think that you also have to drill new
holes in the flywheel to take the later clutch but I am not certain
about this. You could also convert to the E-type flywheel but this may
require a the E-type starter. I think the spigot bushings are the same
but I am not sure.
Having said all of this, I would opt for a five speed conversion rather
than the late Jag box. I consider overdrive a necessaty for freeway
driving. Selling the two Jag boxes will get you most of the way there.
As Doc and Tony say, the BW/Tremec T5 “World Class” tranny has the
proper credentials to live behind a street Jag six. A nice discussion
on the available trannies can be found here:
http://www.jag-lovers.org/e-type/5speed.html
Several people offer bolt-in conversion kits. Buying a kit is a good
idea since the input shaft has to be modified to fit the Jag engine.
For my car, I had the choice between the all synchro conversion or a
Moss OD trans when I bought her. I chose the Moss OD since I am used to
non-synchro boxes and wanted the extra cog. On the other hand, I am
becoming lazier as I age and can now see the advantage of the all
synchro option. I still wouln’t go back to the Jag tranny though.
Regards,
Bill Eastman
58 fhc
PS- if anyone is interested, I can check and see if Bill still has the
all synchro box and conversion parts. He is located in Vancouver, BC.