Also, this is going to look a bit crap for a short while as I'm going to build it as I go along. Please feel free to think of questions that I've not mentioned as I'm sure I will forget most of them

Drifting and motorsport: Should I get a 340/360? What are the drivetrain strengths like for each?
Check out these topics first:
http://www.volvo300mania.com/forum-uk/v ... =5&t=11462
http://www.volvo300mania.com/forum-uk/v ... =2&t=11468
The 360 has a 2.0 VOLVO engine and is better for power and strength but you'll spend more on modifications. Plus insurance is likely to be higher due to the base model being 2.0L. You can't do a renault/valver swap on a 360.
The 340 comes with 1.4 and 1.7 RENAULT engines which is better on insurance when you're young and easier to do the 1.8 or 2.0 16V RENAULT/valver engine swap later on.
You can drift a 1.4 but you will need to weld the diff. If you don't know what this is, please go on DriftWorks and come back when you've found out.
The 360 uses torque tubes which will take pretty much anything, the 340 uses a prop with rubber ends and lacks a universal joint meaning they have to be perfectly aligned. After a Renault engine conversion you will eventually break a prop but it's very simple to just have a spare. Personally I've run my current one for 15,000 miles hard driving and it's been fine. There are no current alternatives to this. However, the 340 is much much cheaper to do an engine conversion on.
What bolts do I need for lowering blocks?
http://www.volvo300mania.com/forum-uk/v ... 13&t=11399
Wait... where can I get lowering blocks?
Get your blocks off Dai from this page:
http://www.volvo300mania.com/forum-uk/v ... f=8&t=1804
Do escort ones fit?
Escort ones DO fit but you'll need to grind off the bit that sticks out
How do I fit lowering blocks?
http://www.volvo300mania.com/forum-uk/v ... =13&t=8890
Are there performance parts for the 300 series? Where can I get uprated shocks, bushes etc?
http://www.volvo300mania.com/forum-uk/v ... f=8&t=1804
What is the wheel fitment on 300 series?
4x100, see here http://www.volvo300mania.com/forum-uk/v ... =13&t=4058
Can I drift a CVT/Automatic?
No.
*Unless you're fritz/superman
How hard is it to do the CVT to Manual gearbox conversion?
Stop thinking about it. Now. One person did it. He regretted it. Sell the car and get a manual.
How much is my car worth?
It's hard to say really but most 300 series go for whatever someone wants to pay for them. As an average guide prices fall between £50 and £1000 for the range of wrecks to minters. It's rare a car falls at either end of that scale and most sit somewhere in the middle.
A modified Valver can go for £1200 but it needs to be pretty special. Just because you put some alloys on your 340, dropped a valver engine in with bike carbs and welded the diff doesn't suddenly mean it's worth £1200. It's not.
I want to do a valver conversion. What do?
Here is a link with some information regarding conversions: http://volvo340.wikispaces.com
You can either opt for 2.0 or 1.8, sporting 150 or 140BHP from factory. The 1.8(F7P) is raspier and has a shorter stroke but lacks the torque of the 2.0(F7R). The F7R has a more satisfying powerful feel and you'll be able to pound it up hills leaving most things in the dust. The F7R drinks like a horse though and you're more likely to break your prop. C'est la vie.
You will be really lucky to get hold of a real Williams engine as they only made 500 of the series I (this is the one you need, series II and III have a chain driven pump). However, you can build a Williams from a 2.0 megane block and the valver 1.8 head.
If you need parts for a Williams remember it's 1992-1994.
F7Ps can be used from any year between 1991 - 1998.
Turbo engine out of the 400 series
It can be done and they go in about as easy as a valver conversion. However many will just put a valver in instead as there are more mods available for them as well as the power difference. But if you want that baaaaaawr PSSssss then it might be the route for you. Make sure you put heatsheilds in though as a 340 burned to a crisp on the side of the M56 last year.
Having not found your answer here, go to the search at the top and enter a few different variations of what you're looking for (such as 1.4 drifting or leafspring bolts). If you try, we are more likely to help you rather than waiting for you to help yourself.