The trans shims should be checked if you take the rear cover off. You need a special puller to get the out put flange off.
That tool is about $80 I think and the tools to do the shims are more. Any chance it can go to the guy that fixed it two years ago? Are you sure it's the gasket?
The oil pressure is regulated by a valve on the front main bearing holder. This is under the timing cover and can be seen here;
The horizontal rod shaped piece above the timing chain controls the oil pressure. You can't read the oil pressure till the engine is running and since you don't currently have this part apart why don't you wait till some of this other stuff is done till you take it apart. Besides you are already doing the engine breather and maybe that will fix it?
No idea about final drive.