Don is right on. Need to have everything mocked up. Fenders too. Messing with the firewall should not be done. Seems like you can figure it out without that if you mock it up and think on it while you look at it all. Also seems like another set of those motor mounts I see mounted to your engine would work upside down for the frame mounts. At any rate, the angle bracket I see should have gussets like the engine bracket, for strength. You need to check trans shifter location and clutch and brake linkage
space. On a V8 there is always an issue with the distributor hitting firewall. Hang the engine in there on the cherry picker and use wooden blocks and floor jack to center/level/locate engine and tranny while you assemble all the rest of the body/chassis parts with a few bolts. Do it all or you WILL have surprises that you don't want. Here is what I don't get: you are putting in the same engine you took out, you are getting all this advice about angles and all that and you say you are maybe going to alter firewall. Aren't you just putting it in same exact location with different mounts? What's all the fuss?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn