Good advice on tires above and 38MM GK SS' are the next set of tires I'll put on my 750 with stock/matrix rims that is mostly used on pavement. That 750 is currently running 42mm Continental Speed Rides which have been great for the few years I've run them though they do better on a wider than stock rim.
Based on my experiments with different tig and lugged multitracks you should be able to run the GK 43mm tires on that 750 frame with sufficient clearance and could go a bit larger depending on the tire and your comfort with small clearances between frame/tires. Fenders, wheel true and wheel dish are factors that could impact how well larger tires fit on the frame and some fine-tuning may be needed.
While the multitrack frames can handle tires over 40mm in width, in my opinion the stock rims are too narrow for them unless you are just cruising and taking it easy. When I run larger tires on the stock matrix rims on my MultiTracks I experience fold-over of the tires when pushing them hard (cornering) unless I run them at a higher PSI than I would otherwise prefer. The loss of rear wheel traction when the tire folds over can be pretty dramatic and if you plan to ride the bike hard it needs to be avoided if possible.
I prefer to run tire pressures low enough to get good flex/traction but not so low that I risk pinch flats. On one multitrack I replaced the matrix rims with Rhyno Lites which are a good bit wider than the stock rims were. On that rebuilt wheelset I run 40+mm tires at lower pressures comfortably. Rhyno Lites laced up to the stock hubs/spokes easily and made for a cheap upgrade without larger changes.
On the stock rims 38mm may be a good max width to avoid the problems with a too-wide tire on a not wide enough rim. Of course actual inner rim widths vary a little through different years/models while different tires, conditions, personal preference and rider weights will lead to different results.