Tall balljoints are an excellent improvement, but I would recomend aftermarket upper A-arms in order to get a decent alignment... with stock upper arms, you really can't get enough positive caster, even with stock parts. With a bigblock, you might not even have enough room to shim the stock A-arms in for proper camber setting either. You could do one end (uppers or lowers) with stock arms for a small improvement, but if you want really good geometry, you'll need to do both.
Howe Precision are the premium quality modular/rebuildable tall balljoints, but at $85/ea you're looking at $340 just for balljoints.
http://www.scandc.com/balljoints.htm
Recently Power Performance started producing a tall balljoint also. These are not modular/rebuildable, but they appear to be a very good quality product for significantly less money. At $50/ea, they're quite a bit more affordable.
http://www.powerperformancemotorspor...?categoryId=29
The best value in aftermarket upper arms is the SPC adjustable arms, at $319/pr
http://www.scandc.com/spcarms.htm
The big improvement is that they change the camber curve, which increases cornering grip by a LOT, as you use the whole tire contact patch instead of just the outside edge. Another benefit is the raised roll-center height, which means you get less body roll. Also, the steer arm moves up in relation to the lower balljoint, so bumpsteer is reduced by about 85% from stock.
With the big increase in front grip, you'll want to take the car out afterward in a big open parking lot and find its limits... Depending on the rest of your suspension setup, you may find that the front now has more grip than the rear, which can be dangerous in a panic stop/evasive situation... This is a big change in overall grip, so some tuning with sway-bars or springs may be required to correct the balance.