1966 A65 Lightning
standard sizes of 3.25x19 front and 4.00x18 rear
only ever ridden it before with K70
thinking of putting Avon Roadrider Mk2 tyres
Firstly, you'll probably find the Roadriders to be good. Mind, I say "probably" because a motorcycle's a physical anachronism, only viable because of the human sat astride it; the rider keeps it viable based on a set of inputs interpreted by his/her brain; there just isn't any guarantee your brain will interpret every input the same as another human.
considering the TT100 tyre but my only concern there is that they don't have the correct sizes that are standard fitment on the A65.
However, don't fall into the trap of thinking Roadriders are "the correct size" just because the ones you've looked at have the same numbers moulded on the sidewalls as K70
's. Or that TT100's aren't "the correct size" just because you don't see ones with the same numbers moulded on the sidewalls as K70
Biggest problem with numbers moulded on sidewalls is they're "section widths" (and, by arcane calculation, "section heights"). How does "section width" relate to the real world? Buggered if I know, and I've been interested in the 'best' tyres for my old heaps for about forty years ...
To decide if your new 'best tyres' (Roadriders?) will actually fit your motorcycle, you need to look at "overall diameters" and "overall widths" - designers of our old heaps weren't generous with spare space around wheels.
When you look certainly on https://www.dunlopmotorcycletires.com/tire-line/K70/
(click on "Explore" | click on "View Specs") and https://www.avontyres.com/en-gb/tyres/roadrider-mkii?cartype=motorcycle
, you'll see K70
's and Roadriders with same numbers moulded on the sidewalls are not actually the same real-world sizes at all ... particularly the widths.
When comparing, also bear in mind overall widths are based on the tyre mounted on the maker's recommended rim width for the tyre size, this will almost always be one width greater than the rims on your bike; in case you're wondering, the WM2 on the front of your bike is equivalent to 1.85, the WM3(?) on the rear of your bike is equivalent to 2.15.
Then even if, say, the 3.25 Roadrider will fit between your bike's fork legs, fatter tyres don't always make a better-handling bike - your bike is light and very low-powered compared to some of the 1980's Japanese bikes that also use V-rated tyres.
Finally here, whatever you fit, risking telling you something you know already, don't use the recommended tyre pressures in a '66 manual
; modern tyres (and that applies to the K70
pressures) start at 28 psi/~2 bar front, 32 psi/~2.2/2.3 bar rear.