The absolute cheapest thing to do is to up the oil pressure and volume.
Right here on this forum, motor oil university classroom 105:
http://www.bobistheoilguy.com/motor-oil-105/
Ferrari engineers say that you need 10 psi oil pressure for every 1,000 engine rpm, peaking 75 psi @6,000 rpm. This formula works in all situations. Do you think Ferrari engineers know anything about going fast AND remaining dependable?
My 3.8 Buick has about 55 psi oil pressure when hot and running 60-70 mph; about 22-24 psi when hot idling, up to 62 psi when cold started. I have a bypass filter setup and BEFORE I installed the turbo oil restrictor jet: http://www.ebay.com/itm/331078883470?_trksid=p2060778.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
I can tell you from personal experience that running the bypass wide open caused a drop in my oil pressure of about 25%, down to about 40-43 psi on the highway. When I put the restrictor jet on and RESTORED my oil pressure, the engine sounds a lot quieter.
I honestly don't know if it is the same for your 3.1; but on my 3.8, you can simply change the oil pressure bypass spring, piston, and gasket; this will increase the oil pressure significantly. I don't have any problems, but I'm going to put this on my own just to sleep peaceably. I already have the parts. For $27.99, it is the cheapest possible thing that MAY help. There are instructions online and even pictures:
http://intense-racing.com/Merchant2/merc...ode=3800_Engine
http://shop.zzperformance.com/store/p/95-Oil-Volume-Kit.aspx
Regarding bearing sizes; factory replacement sizes, unless you plan to remove the crankshaft and have it turned. Good Luck! Keep us posted!