340 Heads Recommendations- Aluminum vs Cast J

If you’re going to go Speedmaster you’re better off buying them bare like you mention. If as an asssembly the springs they come with will be way more pressure than you need with that Crane cam. So now you’d have to buy springs to suit, possibly retainers (if you can’t reuse the ones the heads came with) and pay for them to be checked over, setup. Not to mention if your going to use oem stamped rockers the valve heights on those assembled heads will be all over the place (they were on the two sets I dealt with not too long ago) so you’ll need to get all that corrected or accommodated for somehow) as well as having the shaft saddles checked and possibly corrected.
Or you can just use adjustable rockers and leave the valve heights as they are.

Bare you need the valve job and to ensure the valve heights are the same, set up etc and the saddles dealt with. With that small a cam why bother with aluminum heads to begin with. Just have the J’s reworked, with some bowl work, quality valve job and setup with some matching springs for that cam and roll.