FIRST REBUILD> 225 slant six

Measure the compression first. If fine, no need to rebuild. Oil leaks can be fixed easy with engine in car, without a rebuild. Yours sound typical. The spark plug tube seals are a common leak and the seals are cheap. If set on a rebuild, consider:

1. Remove engine and tranny together, especially if tranny needs a rebuild too.
2. Remove engine, tranny, and K-frame together from underneath (lift body up to roll out), if all need a rebuild.
3. If rings are worn, but cylinders are still round, you can just hone the cylinders with a hand drill and install new rings. I have done this with engine in car (Mopar 4 & 8 cyl).
4. If cylinders are too worn, you will need to take to a machine shop to over-bore and buy bigger pistons.

How long it takes depends on how much you know, how fast and long you work, and what tools you have. I did a re-ring of a Mopar 383 in ~1.5 weeks working some weekday evenings and most of a weekend, sending the heads to a shop.