So here is my 2 cents on "do it yourself". Some things are worth it... and some things not. There is no doubt you can do it. Thousands before you have done it. Whatever it is. You just need to consider "What is the worst thing that happens if I screw this up?" Depending on the repair that can vary.

If you take your engine apart to replace the rear main seal my guess is that you will properly be able to put it back together. With some help and a good shop manual of course. Worse case scenario is things are not repaired and a lot of oil continues to leak. However if you are not careful in reassembly things could go very bad. The up side is you will learn a lot along the way and if you do it properly save a good deal of money. Make a big mistake and you will be looking for a new engine! What you will be missing out on is that a good mechanic might be able to tell you a good deal about that engine while they are in there!

Here is where you get into the fun of an old car. While you have that engine out and torn open there is a list of about 100 things that you should probably look into doing. It's all about time and money. It's not just about the engine... you now have wonderful access to the front end too. See what I mean!

As far as your exhaust manifolds go those rusted bolts can be a real pain in the butt! You may find that you hit them with some penetrating oil and let them soak and then some heat and they come out. You win. But you might need to drill them out or worse have the hole retapped. Better to leave that to a pro. It's worth the few dollars.