You might not want to listen to your tire guy. If anything, wider tires will make the steering heavier. MORE like a truck, not less.
What width rims do you have? Iām guessing that 185mm tires might be appropriate. You may want to look at a specialty brand like Blockley. I love my 185VR15 Blockleys (on 6" Daytons).
Realmā¦and many other C replicas run on 5in x 16 rims either 72 or 60 spoke typically from MWS and 185R16 tiresā¦not a lot of choice in this size and expensiveā¦quite a few owners i know are running Avon or Blockleys or Vrederstein ā¦Steve ā¦see link to wheels MWS
600R16/185R16 looks to be the standard size for XK120/140, and may be correct for you. There are several āreplicaā classic car tires that would be appropriate in this size, though they are not inexpensive.
Photo would helpā¦remember these are hand made diy replicasā¦no 2 are exactly the sameā¦just looked up photos from bring a trailer of your car i thinkā¦left hand driveā¦silver knob on dashā¦lines up with choke cable on webbersā¦if you cant rely on your mechanic then you may have bigger problms to worry about