You don't modify the block, you purchase retrofit roller lifters. Be forewarned though... it looks affordable, but roller conversions can be $1500+ for all the parts. Until you consider the cam, lifters, special length pushrods, tie bars, thrust bearing, different distributor gear, its really a deal breaker. In my case where I build my own engines, it easily doubles the cost of the build, especially on BBCs. My latest build cost me $2000, but the roller conversion would have made that engine cost $3800... just with parts.
The big benefits to roller cams are 1) increased cam and lifter life, 2) greatly reduced friction, and 3) more usable power from the ability to use more aggressive (faster) lift ramps on the cam. Roller cams tend to broaden torque curves which gives more usable power across the RPM range. The first one isn't much of a big deal since most properly broken-in flat lifter cams will last longer than the car, but the other two have definite benefits that can't be denied.
Along the lines of heads, Aluminum heads allow (actually require) a bit more compression to make the same power. Combustion makes heat and heat makes power. Since aluminum absorbs heat faster, they effectively reduce cylinder pressures. That means you can run a bit more compression on the same gas, and the extra compression regains the lost pressure from the heat soaking away. Aluminum's true benefit is in weight reduction. But identical castings, one in aluminum one in iron; the aluminum will actually make a touch less power in most cases. Some guys will also talk about aluminum heads not sealing as well because they expand and contract at different rates than the iron block. But, with a good head gasket and proper installation, they are just like flat cams; they will outlast the car.
For the street, here is what I use to choose heads. Always choose your target power level, then choose the heads with the right chamber size and enough flow to support it (you don't need more, it will just be a ***** on the street) and then choose a cam to compliment the heads. The secret with head flow is to get the flow you need in the smallest port volume you can. For the street, if you find two heads that both flow 250 cfms in the intake port, but one is 230cc and the other is 270cc, always choose the smaller one. That means you still get the same flow but faster which is a good thing. A lot of BBC street builds often use iron 049 factory castings. They flow 230+ CFMs of intake flow which is enough to support 400+ hp. They also have the smaller oval port layout for good velocity. Don't use rectangle port BBC heads on the street. Too much port volume and the flow is great but slow. On flat top pistons they make in the neighborhood of 8.5:1 compression, but very small domes on the pistons will easily take you to 9.5:1 which is about as much as you should run old-school heads with premium pump gas. If you want more flow they can be ported. If you want more comrpession without domed pistons, the decks can be milled.
I promise that wasn't a sales pitch, but if you want some 049 heads I have a few extra sets. I'd be willing to part with a pair, but please by all means do research on what YOU want. Don't let me tell you what you want for your engine
I'm by no means a BBC expert, but I've built some and they seem to stick in my brain more than others; probably because my first engine build was a 454. I'd be willing to help you pick a combo, or help you learn how to pick one yourself.