J-tubes don't seal well if you are using a header and the stock 'asbestos donut' gaskets, so be forewarned that going down that route should include a trip the muffler shop so have flanges welded to the header and j-tubes so that you can bolt them together rather than relying on the lousy clamp for the stock muffler. And the paint that comes on the J-tubes from the factory is for crap...at least that was how my couple sets turned out. It burned off within a few minutes for me. So I would suggest you repaint with something better before you install.
But getting back to your first issue..I ran my '74 like that for a while and saw no major issues with the valve cover gaskets. They seemed to hold up okay for me. The bigger problem, which I -read- about and have not experienced myself, is that the heater boxes can overheat internally and the aluminum casting around the tube that radiates the heat into the air being pushed through can fail if you remove the air supply tubes. If you look at the way the heater boxes are designed, when closed there is an outlet at the top so that some air is still allowed to go through.
If you have absolutely no use for heat at all, I would remove the heater boxes and get the J-tubes, and have them modified for a flange. And make sure you cover up the openings in the rear tin. When I did that a while back I got a few scrap pieces of ductwork sheet metal from the hardware store and cut them into circles, used a small bolt and nut to sandwich two pieces on the openings for the heater tubes and then hit the whole deal with some flat black spraypaint.
Good luck with it.
Chris