a warm welcome from another V8 nut who was already a helping hand on importing two right hand driven V8 the last couple of years from the UK
You get reliable information about the V8 in general and a conversion to manual at the V8-mailinglist at audifans.com - the US guys do that pretty often!
Regarding your problems:
1) Are you sure that the Kickdown works at all? Maybe the switch is broken. That's a quite usual problem. The transmission may still shift back a gear when you open the throttle valve wide enough. The engine should rev up to more than 6400upm when you kick down. Is it?
2) It is usual that the tranmission keeps in a low gear the first couple of minutes after you start the engine. The basic idea is that the engine becomes warm quicker and that the catalysators heat up quicker. The trick with switching to "M" and back again is well-known. Alternatively, you just have to drive faster than approx. 45mph.
3) Transmission fluid (ATF) exchange together with the filter (impoertant!) shouild happen all 40k miles! If never done, do it! You'll have to remove the exhaust in order to remove the transmission oil tray where the filter is located. Also clean the magnets in the tray, and buy a new gasket for the tray. Also be sure to measure the amount of fluid correclty (only when it's warm!). Otherwise you could seriously damage the AT.
4) I'm pretty sure that there has never been an RHD manual V8 - have you ever seen one?
5) Overheating problem: You may double-check your temperature sensors (one located at the bottom of the radiator, the other one at the front of the engine) - when they fail, your electric fan may be disabled. You should also check the metal fuse in the front of the hydraulic fluid reservoir and the reails for the electric fan levels. The electric (auxilary) fan should in any case run when the AC is activated. If it doesn't, it's a fault of the fuse or the respective relay for level 1.
But all that usually doesn't lead to overheating. It's more likely that your radiator became silted up. It may even be that fluid is able to circulate, but it's not really cooled any more. We had that on one of the two UK V8, it was like that car was ran by normal water for ages. A wonder that there were no fishes swimming around in the cooling fluid...
Another not so usual problem is the thermostat controlling the cooling fluid circulation. It's pretty cheap, an exchange is always a good idea.
A cooling fluid radiator flush could also help, in case it's not yet completely silted up.
When you say overheating: How hot was the engine? The PT engine (3.6) is usually not very sensible to overheating, not too much worries.
6) I will be on the UK south coast in October. near Eastbourne. If you'd like to meet... let me know.
In general, what you describe doesn't sound like any serious transmission problem. You may relax
As the UK has pretty strong speed limits and most UK V8s have a pretty low mileage, I guess that most tranmissions haven't been victim to consequent kickdown abuses - good
By the way, the two V8s we've imported from UK can be
found in the Car-DB. The first is from Derbyshire, the second one from Ashford (could be nearby you).
Could you provide some more details about your V8? Interior, color, history, mileage... I'm curious