I'm pretty sure there are 2 different B&M flex plates for the LA 360. One for a 904 and another for a 727. The flex plates are engine specific, so they won't swap from a 360 to anything else without the engine you move it to shaking your teeth out. As mentioned about the exhaust manifolds, be sure to fit the drivers side manifold to the 360 head to make sure it will fit flat at the rear (#7) exhaust port. Most of the later 70's smogger heads have a bump on the head that keeps the early A exhaust manifolds from fitting flush. It's right at the back bottom edge of the drivers side head. If you don't notice it and start cranking down the exhaust manifold bolts, POW......you broke your manifold! If it hits, there might be a set of Doug's D450's in your future!
As far as the drivers side motor mount goes, getting 340-360 mounts from Schumaker will be the Easy Button, but you can weld a triangle shaped piece to your existing mount and drill a hole so it fits the ear on your 360 block. I'll leave a link below about the rubber part of the motor mounts. Everyone with an Early A needs a copy....lol. Good luck with it!

https://earlycuda.org/techdocs/JimBowenTheHolyGrailofMotorMounts.pdf