There are a number of after-market engine blocks (aluminum and iron) now available and even a few that have fallen out of production but are still around. They are all a bit different and certain ones are definately favored from builder's experiences. There is a lot of diffences in after market block weights also.
Since you have some track time in mind, an aluminum block might make sense to you. An FE engine is not particulary heavy for a big block. Compared to a 427/454 Chevy motor or Ford 460 it's downright lean. With at least an aluminum intake to replace a big cast iron one, the weight is really reasonable, especially for a deep-skirt design motor. I've read that some of the after-market cast iron blocks can be significantly heavier than the original FE iron blocks.
Discussing weight, you should know that an ERA is going to weigh a little more than most replicas starting from scratch. Those big, thick main rails and solid-as-a-brick body add about 100 lbs +/- over most comparable replicas. Compared to a Kirkham with all billet suspension it probably is a couple hundred pounds or more. It's a bit of a penalty for what is a very solid and stiff car.
Obviously if money is not an object building up an all aluminum block, head, intake, water pump motor is probably the pinnacle of performance. I think without exception the after market blocks are considered stronger than an original blocks. 427 SO blocks in good condition can be difficult to locate and then they are somewhat limited in re-boring capability, most builders not wanting to go more than .030 over from what I understand. Other FE blocks are still around in fair supply. The 428 KC used for my car had virtually no bore wear - he cleaned it up with a .015 over bore. I felt this was a lucky find and fit my budget.
I would not worry about issues with aluminum heads on a iron block - they have those issues pretty much worked out with modern head gaskets. About the only issue is paying attention to valve train clearance adjustments on solid lifter cams due to the different expansion coefficients.
The "cool" factor is kind of in the eye of the beholder. To some nothing will beat a good old school 427 FE with all correct, period parts. To others, only the latest in modern technology and maximum available HP and torque floats their boat. I fell in the latter area and wanted to use vintage parts - I guess I re-purposed them.
for my 2 cents and from your earlier posts wanting a torquey motor, a nice 428 block with a 4.125 ro 4.25 crank, aluminum heads and intake would work really well. If you want a little more HP and revs then finding a good 427 SO or CO block would be the next step up. And then if you want to knock 75 to 100 lbs off the car and bump the cubes and HP up even further then going with an aluminum block may be warranted.