Buttress or NPTF Pipe threads

Hello Folks

Help me pick the right thread design according to my dimension constraint. I have a body-head thread joint that is fracturing and shearing. The joint can undergo pressures from 2374 psi to 3400 psi. We suspect there is leakage due to an uneven thread engagement. I would like to test buttress and pipe threads but i am not too familiar with either one. Given my constraints, it appears that pipe threads will not fit. Right now, the body OD is 0.544" that can be increased slightly to 0.554" and and the head OD threads cant never go below the OD cylinder of 0.447".

-According to buttress ASME B1.9, I cannot go below 0.5" and can only choose from 20 vs 16 vs 12 tpi? i chose 20 thinking more is better but not sure. Has anyone done Modified threads? whats the process?
-According to pipe ASME B1.20.3, table 3, 1/4-18 will do. Seems quite small but looking at the ext head dia E0=0.47739 & E2=0.5025 and internal body dia E1=0.49163 & E3=0.4670 (good, doesnt touch 0.447")
-which direction pipe threads taper up or down? seems to taper up towards the opening of the body

thanks