If it runs okay, I'd definitely keep the Pontiac 400 in there!
I maybe a bit biased besides my 350 powered Blazer everything I got is Pontiac 400 powered... My Trans Am and GTO's are all 400 Poncho powered.
Love 400's!
BTW Nice solid Blazer to start with. Wish I could find one like that for that kind of money....(I paid over 20 times more than that for mine, and that's without all shipping, import and registration costs to get it here....)