I'm not sure what the hell these other guys are talking about ,, but of course you can weld the mounts ,, either the ones on the engine or the ones on the frame... I have done it many times , to fit engines in cars they didn`t come with originally.. just make sure they are clean and straight before you start ,,, no big deal..
Yes a dogleg 5speed will hold up behind a Z24 ,, they are known to be weak ,, but not so weak that it will fly apart if bigger engine is hooked to it,,,, it wont know the difference.. If you have big tires and peal out all the time the torque from the bigger engine might cause it to wear out quicker,, but that`s kinda up to your driving not the tranny..
And stock.. the Z24 no matter how you look at it has more torque and horsepower than a stock Z20 so why not change if you want to.