Right now i have 90 RS with a 305, and it's automatic. It's my daily driver, but i just want something with more power on the streets. I want to know what would be a better (easier to install) engine for my car. My uncle has a 400 with something like 60,000 miles on it that i can get for around $250 from him or I'm going to look for a used 350. Any thoughts on what is best?

 

Of course do the 400, but you also need to do headers and exhaust, even if you were to just keep the 305. The 400 might do okay with your stock 2.73:1 axle, but would feel even better with at least a 3.08:1 gear, and still return 24 mpg if you get it dialed in well and drive nicely.
You're going to NEED a posi of some sort.
From re-reading your post, I'd suggest finding a '90-'92 V6 / automatic axle for the 3.42:1, 7.625" gears with 28 spline axles, then adding a converstion style locker. Saves you from having to press any bearings or set up a gear set for the correct preload and backlash.

 

If they are both stock motors and you plan on keeping them with the stock top end, the 350 is probably better... otherwise go with the 400 and you have a great foundation to build a monster.

 

Thanks a lot, this was actually really helpful. I'm thinking I might go with the 350 for now, then get the 400 a little later, after i have some experience, time and money haha.

 

that would a complete waste of time if you have the 400ci available for $250 right now.... be smart about this. do the 400ci.. 400 heads are just as good as most of the 350ci heads you'll find... not hard to swap on a set of vortecs anyway...