which plugs now?

[gotmud]

I took the heads from my grandads 68 c10 w/stock 307 to a machine shop to be redone. What I got instead was a set of replacement heads that fit perfectly. All the holes line up etc. He said they were heavier duty and had larger valves in them. Ive gotten everything hooked backup now (except the distributor) and just started to put the plugs in. They appear to have the right size threads but there arent enough threads to get them in more than 1/4 turn. Anyone know how I can figureout what plugs go in these heads? Machine shop owner isnt much help.
Thanks again.
Glenn in Atlanta