If your 350 has #18 heads, it's a '68 ... or someone put '68 350 HO heads on a '70 350. Check the date codes. If they end in 7 or 8, those are good heads. Ausgeseichnet! They might be #16 heads (one of 4 different #16 castings) with date codes that end in 9 or 0. If so, those are "marginal at best" heads.