Can anyone owning a classic truck tell me: does the long wheelbase improve ride or can it also help with handling the trailer at highway speeds? my truck is a 230 inches; what would change if I were to pull the same 53 ft dry van trailer with - let's say - 260 inch truck? thanks!

Typically, the longer the wheelbase, the better the ride.

You would improve your ride a bit with a longer wheelbase, but you might lose some fuel mileage. You would be creating more of a gap between your truck and trailer. You would need to fill that space with something. A larger sleeper, or longer windbreaks on the back of the cab.

It's clear how the wheelbase affects the ride; what I'm curious about is will the longer wheelbase make the truck hold the road better with a 53 foot trailer? I mean, right now when I'm driving my 230 inch International I have to steer - I think - more than the guy in a Pete or Wester Star 4900EX. The end of my trailer wiggles and because of the short wheelbase it pulls the truck left and right .... I believe it'd be harder for the trailer to affect the direction of the truck if the said truck had a long wheelbase. Any comments?

I don't think the trailer will fishtail or move back and forth due to the length of the trailer. One of the trucks I own is an International 9400 Eagle. I don't have a problem pulling a 48, 53 or over-sized trailer. The pulling from side to side is likely due to you moving the steering back and forth. You need to concentrate on keeping a steady hand on the wheel. It doesn't take much steer movement for the trailer to move back and forth. I have noticed some drivers who constantly move their steering wheel back and forth. I try to keep a steady hand with as little steering movement as possible.

yes some of the short wheel base day cads pulling ten axle king B's can be a real hand full, you can feel the trailer just jerk you around, the longer wheelbase will do away with that. the only way to get from that is to pull double 53 footers then the one counterbalances the other.
OK you have a problem which is unique to internationals, it is two fold, the cab mount is way to narrow at the back, a lot of the steering you are doing is from cab movement.
two the drives set up is different, thy do get what is called push in corrers for some reason is more noticeable with tandems than tri axles. just some things I have found driving different trucks with different trailers that's all folks.

If you're having steering issues you need to look at; alignment, kingpins, springpins, tie rods, drag link, steer gear, steer gear u-joints, rear spring eye or trunion bushings, trailing arm bushings (if equipped) trailer suspension and bushings also is the fifth wheeel geased, cab mounts can cause sway if they're loose, tire pressure can be an issue, tractor or trailer ride height can in some cases cause a problem. Tractor wheelbase should not be an issue in a standard tractor trailer situation whether it be a 48 or 53

I'd personally stay away from long wheel base tractors unless you're doing something specialized.