Biggest thing I do is clean the block as clean as possible. I've built quite a few and most have lots of casting sand and crud. Then, when you get it together set the timing properly. I'll have some truck fans at Hershey
You might do a search on using vinegar to clean everything out. There has been a lot of discussion that when you build a flathead, you need to stand it on the front end, poke the water p***ages, pressure wash, beat the block with a rubber mallet to dislodge the sand and crud to fall out of the water pump holes. If you missed the opportunity to do that, then flushing it out may or may not yield success. There has been a lot of discussion about the merits of different radiator flush chemicals, too. The times being what they are, a lot of the over the counter chemicals are not as potent as they used to be.
well i just bought a 351 w....that'll slove my problem and i'll have enough hp to run air !! but thanks to all