are any of inline tubes flares leaking at the block ?

steel or stainless lines ?

with stainless lines I have had luck with a SMALL
dab of anti-seize to the threads of the fitting only and cycling the fitting 3 or 4 times at the connection to get it to seat fully - anti-seize lubes the fitting

Just helped a friend with a 61 Vette - he bought all repro stainless lines and EVERY connection leaked - got them all to seal with the above method

Never had a issue with OEM block - repro blocks yes

I would also verify 1 of the inner seat's on the block isn't damaged