I dont think its sketchy but be warned, thats a pretty good sized cam and from the sounds of it stock pistons so probably less than 8:1 comp. If you buy it youll likely want to put a much smaller cam in it or swap the pistons out.
40 thousandths off the heads and 8:1 comp? An 81 5.9 comes with ADVERTISED 8:1 stock. Realistically speaking it probably had 7.1:1. milled that much would jump to around 10 or 11:1 comp.