Okay, the Weber DGEV 32/36 is a simple installation that uses the stock Toyota throttle rod which connects to the rear of the carb. You just attach the fork shaped part that comes with the carb kit to the back of the carb, and the throttle rod fits right in just like stock. Thus the kick down cable remains the same. If you go with side-drafts or other carbs, I don't know. Weber probably includes the correct parts. Order direct for Redline Weber. Speak to Bud Pauge for tech issues there.
Regarding your exh manifold, I had mine welded and it has held up very well. It was welded by a professional, experienced diesel mechanic who knows how to weld cast-iron. I believe he brazed it with stainless or bronze, I forget. Or you can get one from
Toyhead Restoration. He has one that supposedly will not crack. Unless your going performance, I wouldn't bother with a header because it probably won't do that much for you. There is a 20R head and early-22R block combination that supposedly gives you more power and you can added all the goodies. Apparently the 20R head flows better, but the 20R exh manifold will not work, so you must use 20R part. Weber makes a 38/38 downdraft and then there are always the side drafts, depends on how deep your wallet is. Good luck.
BTW, I use to have a 71 Datsun 510, but I have a 71 Z now.
