As for the gearbox, i have no idea. I'd tell you to stick an auto in it, but that would get me crusified here, and is no good for circuit racing. As for the diff, GT capris were available in a few different ratios. I just sold one with a 3.23 centre in it, but you can get higher. I cant remember which year it was, but a similar vintage BMW had the same diff in it as a capri, and the gears are interchangable. Ring any good diff shop, and they would be able to tell you which cars had that diff in it, and then go around the wreckers looking for a suitable donor diff.
To get more power out of you engine, get a set of D-Port heads. These were the heads used in the mid 70's on the mk1 and 2 capris for racing. Very good flow, for their time, and would get a lot more power out of you engine. Be careful when you get your current heads machined, as you want to make sure you have enough piston to valve clearance. If you take to much off the heads, you will start bouncing valves off the pistons. Best to get a set of lumpy top pistons to bring comp ratio up.
Go to www.burtonpower.co.uk
They have the biggest range of after market parts for english cars in the world, and will ship overseas.