You see people making all kinds of various numbers with the 7M.... I'd say that the majority of owners fall between 250 and 400 at the wheels. I know, that's a rather broad spread but it's accurate. There are plenty of people who push their luck and go for 400+ without rebuilding that 20+ year old motor and end up making 0 RWHP.
If this is your first Supra, but not your first hobby car and hopefully not your only car then fine.... But, if it's your first car and you expect to learn from working on it you should count on a lot of painful lessons and perhaps consider a better supported platform to begin with.