Hey brother, the old Zed car eh?
I had an old Vauxhall Velox and it was a pile of turds in comparison to your fine number. Last month in Northland in the top of the North Isalnd I was offered one and had to pool all my reserve to say no to it.
First rule before throwing money at it is to have a static leakdown test done at a reputable engine shop. They will presurise the cylinders, and measure how much sealing there is in the head, valves and pistons. If there is less than 80% retention over the industry standard time scale, then save up for an engine rebuild. The six won't like any added pressure!
The secrete with the Zephyr is that it just loves carburation because it revs well and won't blow up like some other cars of the era. My suggestion is to consider fitting a water injection unit holding 50 % methonol, 3% methel ethal keytone, and the rest water. This can be primed to come in action when the manifold pressure drops below 20 kPa. Talk to an automotive electrician. Then you can run upwards of 11:1 compression if you can weld or shave that much. Follow David Vizards discussion on page 119 to 124 in his Ford SOHC (Pinto) book (1984 - Fountain Press 629.2504 VIZ). Another book by the same author is Tunning BL's A series Engine (1985, Haynes629.2504 VIZ). Page 206-213,325,326 and 474, 475 will ensure you don't reduce you ride to a pile of scrap! Compression and the proper ignition will boost your power over 10% if you use methonol in addition to water. Part throttle fuel economy will skyrocket! If your in the US, you may find these books hard to get, but they are worth it! I noticed there had been some discussion around the room on Water Injection before, so don't just listen to me, check it out yourself!
If you like, the huge compression hike can then allow a long duration cam to survive the rigors of street driving, without being "cammy". The old 268/288 grind BMC master was very good for this engine 20 years ago, but things have changed a little since then.
After trying to add Triple SU carbs to my old Vauxhall, and then seeing how simple the porting was in the Mk3 Zephyr, I'd instantly go to handbuilding a triple carb intake out of the right diameter steel tube, and hook up an external heat source to aid heating if you want good cold start emmisions. Best carbs are Stromberg CD 175'S off a Jag, Rover or perhaps the carbs off a late emmisions MG. If you can't find any don't worry, but these are superb items if set up properly. I had about 20 carbs in my basement after a local Mini dealer had to get rid of his stocks...for free! Jack Collins is using these on his Locost 7 with a 200 I6.
The gearbox was found in the later British Capri 3000 GT's and GXL's. It had a reputation for being tough and dependable. It also had the same spline count as the old Y-block Ford V8's. If you are in Australia, England or New Zealand, they can be found but are expensive. Alternative is modifying the UK 4spd Cortina/US Capri or 5-speed UK Sierra trans to suit. A friend of mine has redrilled the standard bellhousing to suit a Toyota Hilux/Celica/Supra Five Speed. It wasn't too hard, and he did it on a shoe-string budget.
My suggestion is keep being realistic like your doing, and remember, you can botch up three times and still pay less than a professional job if the risks are low. A car nut is always someone who can do for 5 cents what any darn fool can do for $10.
I don't have much money to spend
