I am assuming you mean to replace the aprons from the core support to the firewall? This is not a job for amateurs, it's is quite an intricate and time consuming job. Why are you wanting to replace the whole assembly?

If you cut the aprons off with the engine in, the frame will sag. This type of repair is best done on a bench with 3 dimensional measuring. You can try to do it without, I would suspend the car by the pinch welds on the rockers, 4 corners. I would then block up the frame rails in front of the a arms and behind the wheel. I would then measure across the top, X the opening with the fender bolt holes, note that for future readings. Also measure across the holes. Only cut one side at a time.

The best seam sealers are epoxy 2 component that need special guns. Evercoat has some that work in regular caulk guns but never tried them. Most cheap seam sealers suck big time. Any auto paint store will have a full line of caulk. I like 3M 8308, it's $30 a tube.