1250*F is accepted max temp for sustained driving... there are a couple of factors to keep in mind here.
First, the engine you're running is using the larger injector bodies (9mm) compared to the later (91. 5-93) engines... as a result the head had a tendancy to crack between the valve seats due to heating /contraction. (less material in that area to accomodate the larger injector bodies)
That means extra care is needed to keep that problem from becoming a big issue for you... so, keep your temps lower than even 1250*F if you can.
Second, the max temp of 1250*F max is ok as long as you keep an eye on your coolant temps during sustained periods. Eventually it is possible to see your coolant temps climb because the engine/head etc becomes heat saturated and the radiator/cooling system can't keep up.
Watch those issues and you should be ok.
pb.