With the Block5 version of Falcon 9 booster soon to be flying, will they be used for the Falcon Heavy, or will they remain a stand alone thing?
Yes, the first Falcon Heavy flight was really not a great test of the concept, since it was using Block 3 side boosters and was a one off center core.
The plan is to use Block 5 boosters for all components. Two side boosters, center core, and second stage will all be built to the Block 5 on the future Falcon Heavy boosters.
There will be differences between the side boosters and a standard Falcon 9 single stick, but mostly minor. Consider that in the first case of a Falcon Heavy the two side boosters were used Block 3 boosters, modified after the fact to be side boosters. This indicates that the differences are not deeply embedded in the design if it can be easily modified.
The center core will be a different design, strengthened to support its mission and Gwynne Shotwell has said that they will make it as a different 'model'. But the engines, and overall standard will be the Block 5 approach.
I think one important point to clarify in this context is that Block 5 is sort of a standard for a bunch of upgrades to the Falcon 9 first stage booster as opposed to an exact vehicle design.
The side boosters will be very close to that, with minor changes for use as side boosters, nose cone for example.
The center core will be a different vehicle, but still following the Block 5 standard, just modified for a different mission.