Hi Tin,
I agree with the others, you can't splice plug wires. However, you often can quite easily shorten them for a better fit.
Hate to tell you this, but I suspect those red MSD wires that are on the car in the photo are far superior to the "green" ones you got from TRF, which are likely lighter gauge "standard" plug wires. If originality isn't a concern, the MSD wires might offer smoother running thanks to a stronger spark at the plugs (that might allow a slightly larger plug gap) and less voltage drop or "leakage".
The MSD wires appear to be 8mm or 8.5mm and might be a "universal" set that can be sized quite easily. I much prefer to cut my own wires to length, since the "ready-to-fit kits" are usually a little long and IMHO it's a good idea keep plug wires short, so the juice has as little distance to cover as possible. Also, some of the ready-to-fit kits come with proprietary ends that are heat molded on or otherwise made impossible to change out or resize.
If the MSD wires are like other "universal" wires, they often have crimped-on ends and it's just a matter of slipping off the rubber boot, carefully removing the crimped-on end and shortening the wire to within about 1" of the length you want (short as possible, but with a little slack so that there is no strain on the wire when the dizzy is turned to set the timing). Next remove the insulation from around the core, exposing about 1/2 to 3/4" of it and bending the core back over the wire so that it makes lots of contact with the metal of the crimped-on end, Then re-clamp that end in place. Slip the boot back on and you are done.
Some other wire ends use a sort of screw with a pointed end to pierce the wire and make the electrical contact. These are even easier to work with.
In many cases, one end of the "universal" wire is pre-installed and might not be removeable, while the other is up to the user to install, can later be removed and re-installed. If that's the case, it's usually the spark plug end that's pre-installed and permanent, while the end of the wire at the distributor is removeable.
You can even buy rolls of plug wire and various types of ends to build up your own wire sets from scratch. The only brand I know of that seems to require expensive, special tools is Magnecor (And that might have changed since I last bought a set. But the tools were expensive enough it worked out better in my case to make an exception to my usual procedures and just buy a ready-to-fit kit for my Land Rover).
Currently I'm using Mallory 8mm wires on the Triumph, but have used various other "universal" brands on this car and elsewhere, all of which offered some means of shortening and fitting. In fact, I usually buy a universal V8 set, ending up with enough for two complete sets of wires for the 4-cylinder engine. V8 sets are often more easily found and this can work out a lot cheaper per set.
I do recommend some sort of wire separators be used between the wires, to prevent the wires from touching each other where they run relatively parallel. When that happens, it's possible the voltage will jump from one wire to the other. This doesn't occur in most cases if the wires are crossing each other at a 90 degree angle, approx., even though they are touching. But I still try to avoid wire-to-wire contact(or wire-to-grounding metal anywhere on the engine, for that matter) whenever possible. Modern upgraded 8mm, 8.5mm and 9mmm wires in particular are better insulated than older types. But still capable of leaking voltage from one to the other, which can cause a nasty miss in the engine. This is especially true with modern, really high energy ignition systems (TR6's stock system is much lower voltage). If you want to see a simple type of separator being used, please feel free to follow the link below to some photos of the dizzy and wire installation on my TR4 (In the "Carbs, Engine & More" area).
From your photos, I can't tell what kind of coil or dizzy cap you've got, but those might have been upgraded, too, since the plug wires apparently were. (In fact, the entire dizzy might have been replaced, although I know that's not so easily done on TR 6-cyl. motor... The only replacement I know of is a Mallory dizzy that doesn't have the tach drive, in which case the tach needs to be converted to electronic either by replacing it or by changing out its innards to keep the original appearance.)
Oh, and when you install the wires, suggest you remove one at a time, do any shortening you wish and replace it, to keep from mixing up the firing order (personal experience here... and more than once!). If you bought the car non-running, it would be a good idea to double check that the existing wires are installed in the correct firing order (per the manual), before starting and re-sizing work.
I also think it's a good idea to use a little dielectric grease on all ends when fitting to help keep moisture out, prevent corrosion and make the rubber boot easier to remove when the time comes. This special grease for this is available in small squeeze packets in most auto parts stores, or in larger tubes there or at some hardware stores.
Hey, if you decide to go with the TRF wires and really don't want those MSD wires, send em to me! They look almost new!
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