I don't think mixing oil and coolant will be a problem. One of the nice things that comes with a chevy small block is the interchangability of parts.
If you want original looks with script valve covers, oil filler tube, etc., you will want earlier heads with offset valve cover holes too. Buy the book Chevrolet by the numbers book by Alan Colvin. Although numbers matching may not be what you are after, the casting numbers that you are looking for would be reletively cheap and quite available.
