i havent messed with a 6000 in a while, HOWEVER, just using the larger 98 antenna for rain fade and signal quality issues is absolutely fine. you should be able to use the 1watt transmitter from your 74m dish on the 98 without any issue. however, i cant remember, and im too lazy to go get one out and look, if the 6000 allows you the option of selecting transmit power during registration (with latest software of course). if it allows you to select 1 or 2 watt, you CAN use the 2watt transmitter, selecting 1watt power. adversly, the modem already registered as a 1watt system, should be fine without having to reregister (i just wanted to make note of where the info is coming from for those reading this) the 2watt transmitter will in effect, be used as a 1watt transmitter.
tech support would agree, ive run into similar issues and spoke with them regarding this. as long as the MODEM has the correct power selected based on modem and power supply, you can use a 1 or 2 watt transmitter without an issue. however, you obviously cant use a 1watt transmitter selecting 2watt during registration.
also, if you could get your hands on a 64watt power supply (if not already) then you could reregister under 2watt (again if the 6000 allows, i dont recall). then not only would your receive hang in there longer, but fewer failed transmissions during inclimate weather as well.