You have a lot going on here. Not sure either one requires removing the engine but you may have more going on than just these items.

You said your engine leaks? That can mean many different things? But since you list a few I'll give it a shot.

You have many exhaust manifold options. You can replace with stock log type which will not require any modifications to your existing exhaust. You can also upgrade to longbranch set as you mentioned. They are a nice upgrade but not cheap. You may need an oil filter re-locator and need to adapt to your current exhaust. You can also go with headers but they have their own set of issues and may require additional fit work. You should gain a few horses with longbranch or headers.

As for the oil pan... I can't tell if you are talking about the drain plug or the bolts holding the pan in place but either one should be a relatively easy fix for any mechanic to retap and not require a new pan unless the drain plug hole has been already repaired before now beyond repair. They even sell oil drain plug fix kits just for that occasion.