SB-201 connection to 6300

David, check out pages 33 and 37 of the Series 6000 hardware manual this will probably answer any questions you have.

TX RELAY OUTPUTS [1,2,3] TX1, TX2 and TX3 (TX on FLEX-6300) are outputs for keying external equipment such as amplifiers, external T/R switches, etc. They are all individual outputs, isolated from each other. Refer to the SmartSDR documentation for explanation on how to configure the timing for each output. – The TX Relay outputs are designed to handle signaling levels of up to +40 VDC @ 140 mA maximum. Some older amplifiers exceed this voltage level and damage to the radio can occur if directly connected. Verify the voltage/current on your external device before connecting to TX Relay 1-3. If there is any doubt or the levels are too high, use a buffer/isolator box between the radio and the external device. 

http://www.flexradio.com/downloads/flex-6000-hardware-reference-manual-pdf/

David, if you have the softkey mod in the SB-201 you don't need a relay between the rig and amp. That's the whole purpose of the softkey mod. And don't bother with the ALC connection - you don't need it. Hook the TX output on the 6300 to the Ant Relay input on the Heathkit and make sure RCA TX1 is enabled in the radio setup under TX.
73,
Doug K4DSP