Heads are a mystery to me ,but I found this info that may be related.
The introduction of the 273 in 1964 heralded the way for the thin wall casting 318, 340, and 360 engines. As most of you know, these small block engines were loaded with performance potential. With the new block design came a new wedge head. By 1968, performance had reached a new level: the legendary 340. The well-known X-Casting, with 2.02 intake and 1.60 exhaust valves proved to be the key. Gone were the poly heads and heavy blocks. Small block performance per cubic inch was on par with the big boys.
The 68'-71 X-head had large intake valves, 65cc closed chambers, and ample intake and exhaust runners (ports). They are considered the premier performance bolt on for the small block engine. The downside is that they command relatively high prices at swap meets. and are usually in need of work. The last X-head was produced almost 25 years ago.
Along comes the next generation of small block heads the common 340-360 head the " J ". (I have personally seen early J heads with 2.02 intake and 1.60 exhaust from the factory, although the books and expert say they were never made.) 99.9% of these heads were garden variety 1.88 intake and 1.60 exhaust. Chrysler was downplaying performance by 1972 and the 360 was about to take the place of the 340. (the last 340 was produced in 1973 and the first 360 in 1971 .... allowing for a three year overlap.)
This is not bad news. J castings are plentiful and inexpensive. The small intake valve is actually an advantage. Why? When you find a set of heads and take them to your favorite machine shop, have 8 new 2.02 intake valves in your hand. When you enlarge the seats for the oversize valves, it will be on fresh material, and will not sink the valve in the seat. As for the intake and exhaust ports or runners: every 340 and 360 head has identical port configuration. With a good 3-angle valve job, minor pocket porting and cleanup you will have a very good street-strip head. After 1973, most 360 heads had hardened exhaust seats installed for today's unleaded fuels. This is an added bonus for street performance and reliability. The 68cc head will be approximately @c after milling for straightness.
The T/A head
The T/A heads have identical porting and runners as any 340 or 360 head. In fact, the head carries the 360 J casting number. The intake rocker was offset and the pushrod hole was elongated (to provide provisions for porting). There is no performance advantage to these heads as cast. In addition, not one of these engines was every produced with 4-bolt main caps. They did have provisions to install 4-bolt main caps (a thicker main web and filled pan rail.) Do not spend big dollars on identical heads. Leave these for the people who need them for a restoration. Note: when I say not produced. I am referring to production line vehicles.
Remember:
* all 340 and 360 heads have identical port size and configuration.
* installing 2.02 intakes is simple and inexpensive (they all have 1.60 exhausts)
* the heavy duty stamped steel rockers and shafts of the small block (hydraulic) are good and stable to 6000 rpm, and are maintenance-free.
* the exhaust flow side of the small block head is the weak side. This is why the small block responds so well to headers or the early 340 exhaust manifold.
