Roller rockers work great in this application. I ran a set of Comp Cams Magnum 1.6:1 roller-tipped rockers in my old 2.8 with great results, and my 3.4 has a set of eBay El-Cheapo Small Block Chevy V8 1.6:1 roller-tips that work as well.
Note that unless you're using the stock GM camshaft, you should probably upgrade to a performance set of valve springs & retainers. The stock springs might tend to coil-bind with higher lifts.
Roller-tipped rockers will reduce wear on the valve stem and valve guide, due to reduction in the 'scrubbing' friction of stock rocker arm to valve stem. Plus they'll reduce oil temperatures and add a 'smidgeon' of extra hp, just by virtue of the roller tip.
Regarding the Q of distributor, install before or after, makes no diff; you just want to make sure you line things up to where the dist is either at TDC on compression of #1 cyl, or align to TDC and compression stroke of #4 cyl. If you have the valve covers off adjusting valve lash, it's easy to watch the pushrods go up and down and figure out whether #1 or #4 is on its compression stoke when the timing mark on the harmonic balancer is lined up to "0" on the timing scale. And of course, point the rotor to whichever cyl you line up to, #1 or #4.
Regarding proper setting of valve lash, don't get it too tight or them motor won't start!
For any particular valve under adjustment, make sure the other valve has started to be actuated; this means the lifter of the valve you're adjusting is, by definition, riding on the base circle of the cam.
Tighten the rocker arm locknut while "jiggling" the pushrod up and down. You'll feel the amount of clearance reduce as you tighten the locknut further. When you get to the point where the pushrod no longer "jiggles" up and down, you've reached the point of Zero Lash.
From Zero Lash, tighten no more than an additional 1 turn. Regardless of what is usually said about 1-1/2 turns past Zero Lash, that's too much and may result in a no-start condition due to the valve being held open.
I run my 1.6:1 roller-tipped rockers at 3/4 turns preload and they work just fine.
Regarding the rocker balls that don't fit, I bet if you slid them down a 3/8" stud they'd fit just fine! The factory probably slipped a few V8 rocker balls in there.
I wouldn't recommend using used ones as they'll cause additional wear on your nice, new rocker arms. New balls are cheap and readily available at most any auto parts store.
Just ask for some for a 1990 Chevy S-10 with 2.8 V6, and you won't confuse the Parts Dude!!
G'luck with your performance mods, I think you'll be pleased with the results, once your computer "learns" the new parameters.
You're going to want to check/adjust minimum idle air speed and T.P.S. voltage after any performance mod such as this, as it affects airflow thru the engine and will throw off the stock settings.
If your rig idles or runs strange after the mod, that's the reason for it.