Redesigning/tuning a coil

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Original poster: pepperman-at-SoftHome-dot-net 

Hello, I'm new to this forum;  I've been devouring backposts from
the archives lately, and I really enjoy this valuable resource.  I
recently was offered the chance to tune up an existing Tesla coil,
and I jumped at the chance...I would like to build one soon, but time
and resources may not allow me to, so this seems like a pretty
good opportunity.  However, I have to get as much done as I can
by Wednesday, because I'll be leaving the area then.  I've checked
out the coil, and am making some recommendations to my contact
on what to change; I'd appreciate any comments to let me know if
my ideas are on-target, or if you see something you think I should
change.

The current setup (all inch measurements are approximate):

15/60 NST

Primary: about 19 turns of insulated wire, I'd guess around AWG 12
or 14, 18" diameter bundle

Secondary: 8.75" OD, 36.5" wound length, 1500-1550 turns of
AWG 22 varnished wire

Topload: currently either a metal bowl upside-down over the top or
a 8" x 23" toroid of aluminum flex ducting

main cap: 16.2 nF (6 caps in parallel, each 2.7 nF -at- 40 kV--the
doorknob type, probably ceramic)

Spark gap: a single gap using several nails

Ground: I'm not sure if he has a good ground--I don't think so

My main ideas for improvement based on what I've read here:
-switch the primary to 1/4" bare copper tubing, around 12-14 turns
to allow for plenty of flexibility when tuning, in a cone angled at 30
degrees for better coupling
-build a new toroid, 6" x 26" (aluminum flex ducting)
-main cap: see if I can persuade him to try something else, since I
know the doorknob caps are really lossy and their C changes with
temperature
-spark gap: an RQ/TCBOR style multiple spark gap
-connect a good ground

Main questions:
-Will the 6" x 26" ducted toroid be too big for this coil?  I was also
considering 5" x 20", but someone my contact talked to said it only
comes in the even sizes, e.g. 4" and 6" but no 5", so I figured the 6"
was better than trying 4".  I thought this was strange, since many
people have mentioned 5", but perhaps they weren't using ducting.
Any thoughts on how appropriate this size is?

-How acceptable are the ceramic doorknob caps?  I know they're
not the greatest, but I'm thinking we may just stick with them, since
we already have plenty, and replacing them would cost a fair
amount.  Also, the current C seems pretty reasonable; any
recommendations on either front?

-The coil is currently located underground in a large warehouse-
style area (though I don't know if it'll stay there), so driving a
grounding rod isn't feasible--would connecting the ground to
structural steel members be acceptable?  (I don't think the water
pipes would be a good idea, as they're connected to the normal
electrical ground--is the structural steel also connected to the
electrical ground?)

Any other thoughts/comments?

Thanks!
Michael Johnson