The 277T was designed for the T50 and T70, as an automatic (photosensor) flash
which puts the Program mode camera shutter at a flash-compatible speed and sets the aperture on the camera.
which with the 'F No Set' allows you to freely choose f/number to use (between f/2 and f/22).
The 300TL flash is a TTL metering flash, in which the camera hotshoe has a contact that permits it to tell the unit to 'flash stop' when the meter in the camera detects 'enough light'.during the exposure...the contact is missing on the T70. The T90 was the first Canon camera to support through-the-lens (TTL) flash metering.
But you can set the 300TL in manual mode, even in two output settings, and use it on your T70, set in manual itself.
You then have to set the camera shutter speed at 1/90 or longer and the aperture according to the table on the flash. If that table is missing, look it up or calculate the aperture setting from the respective guide number, for the two power settings.