XS400 Road Tyre Replacement

[Markfb]

Good morning from rainy Glasgow! I’ve been all over the place trying to find matching road tyres for my XS400. Rear 3.25/16 and Front 3.00/18, I’m aware stock conversation of Rear 120/90 16 Rear & 90/90 18 Front but for love nor money can I find matching tyres, tubed, for the bike!
Any ideas UK based where I would find any luck? Is it imperative tyres are matching? I’m aware there will be variations depending on brands/grip but I’m at wits end! Any help greatly appreciated.
Bike attached, a lot of blood sweat and tears. Looking to go digital next for speedo/rev so any pointers in that direction would be great too!

 

[CaptChrome]

As far as matching tires, the only company that makes a tire model that is in the stock size range for the XS400 with 18" front and 16" rear wheels is IRC. They have two tire models that will work:
Durotour RS-310 (90/90-18 front and 120/90-16 rear)
GS-11 AW (3.00-18 front and 4.60-16 rear)

I have a set of the RS-310s on my bike and have 5000 miles on them. They work well and I would consider buying another set when the time comes. The rear profile is flattening a bit in the middle of the tire as expected, but I think I will easily surpass the 6000 mile mark on the rear tire which is my benchmark for life expectancy. However, I don't have much wet weather experience with them as I don't try and ride on a rain forecast. So, my time has been on mostly dry pavement and some sand and gravel roads. IRC is a Japanese company and their tires are manufactured in Japan.

I expect that most people mix front and rear tires from different tire models as well as different companies.