My wife's 1991 745 has one of those rear-mounted camshaft driven distributors; my 240 parts cars do not. The engine has 300,000+ miles and has been weak and missing going uphill for awhile. The front camshaft oil seal has a leak I had not noticed and the upper waterpump seal has now failed as a result of this neglect. Since I am looking at repairing the waterpump leak, and oil leak, and probably ought to replace the timing belt: I'm thinking about pulling a similar head from the junkyard and swapping them to improve the power loss I presume is mostly due to excessive wear.

Money & time are big problems for us and I am needing to keep it cheap & brief.
I'm kind of personally overwhelmed right now questioning my judgement & looking for advice because I am worried about missing something obvious that I should have thought of in advance and thus creating more problems for myself & my family. Any tips about the camshaft or whatever?

Would a good used engine from a 240 (with the other common style distributor) also mean changing the ECU and??? I do have a relatively intact 1990 245 (it was the best running Volvo we owned but it got "rolled" and unsafe to drive due to body damage) thanks