how much power do you need??

Staying with your desire to autocross i really dont think an iron block, let alone iron heads (whatever the enginetype) is an option,

an alluminim ls style engine can be safely and easily built to about 420 cubic inches.. the limit is arround 500 cui.. but a steetable 427 ls will put down in excess of 700 hp with no poweradder and a decent idle..

yes you can buy 700+ cubic inch big blocks with aluminum block and 1200 pump gas hp, but it will set you back 40.000 dollars, a procharged ls engine will get you close to 1000 hp for half that money if you spend it wisely

if you want a big block.. get it, if the big block of your dreams is an ironblock motor with a stock crank and recon heads.. do by all means buy your buddies engine.. but if you intend to put in an ls motor at some point in time, then you are just wasting money messing with the big block