What cam did you go with? Maybe more important, what heads? The problem is, at 4200 rpm, the new motor isn't making any more horsepower than the old motor. The stock heads on an 88 block probably can't keep up with a 383 past 4200 rpm and that is why you are loosing power up top. Consider bolting on a set of Vortecs if the compression ratio stays reasonable (the cost the same as a new prop).

And, 4000 rpm is too much to cruise at with a small block. 3500 is about as high as you want to go for cruising speed.

I own a 21 ft boat with a 350 and a Volvo sterndrive. I may be building this boat a motor just like the one in my truck in the future. Currently, I get 28 mph at 3500 rpm and 40 mph at WOT and 4200 rpm. I've got a 15x19 prop.