Here is a link to a 23 page report for a TC head flow bench study.
http://neverworld.net/socalnetscum/files/tech/HD_TC_IntakeFBStudyRel1.pdf
Here is the summary of the differences between the heads. (If you want the flow numbers you will have to read the report.
’99-’04 TC Head
The heads on TC engines from 1999 to 2004 were from the same casting. These heads
came with 5/16” valve stems with 1.85” intake valves and 1.565” exhaust valves
controlled by coil springs with about a .510” cam lift limit when using stock rockers.
The last digit of the casting number for these casting heads is -99.
’05 TC Head
The heads were from the same casting as the previous years, but they came with smaller
7mm valve stem with 1.805” intake valves and 1.575” exhaust valves controlled by beehive
springs which could accommodate higher than .510” cams. Although, there is no
additional flow potential as cast with the greater lift ability.
’06 and up TC Head
Starting the 2006 model year, the head casting changed. The intake and exhaust ports
were re-shaped and the stock head flow potential was improved from the earlier -99
castings. These new castings were paired up with the smaller 7mm valve stems and beehive
springs introduced the previous model year. The intake manifold mounting area also
changed between the -06 castings and earlier. To use these heads on pre-2006 Twin Cams
newer style manifold clamps must be purchased.
With the exception replacing the old style intake manifold flange to the newer symmetric
flange 26993-06, the -06 casting heads are a direct bolt on as a replacement for the
previous castings.
I hope that clears things up for you.
Randall