There is NO 30amp spoolmatic. There is a 15a and a 30a, these are model numbers and do not relate to amps. The numbers refer to the length of the leads. That is to say with a 15a you can get fully extended in a straight line about 15 feet from your machine and with a 30a you can get roughly 30 feet. The spool gun itself on the 15a and 30a is the same gun only the lead length is different.
Further you will not be welding at 400 amps at 40% duty cycle with a 300DX Shopmate because the duty cycle of that machine hits 100% at just under 200 amps and falls to about 25% at 400 amps, the machine's maximum output. The spoolmatic 15a/30a has a duty cycle of 100% at 200 amps which should just about perfectly match the 300DX's capacity.
The spoolmatic 15a/30a is considered by many to be the BEST spool gun available. If when you read the specs on both the Shopmate 300DX and the Spoolmatic 15a/30a you still believe you need 400 amps I would suggest that neither is up to what you want to do and you should consider a much larger power supply and probably a push/pull system. Good luck.---Meltedmetal