According to SAAMI standards, the 3 1/2" 10 has a service pressure of 11,000 psi. From some old information I have, back when the 10 was still 2 7/8" (when your Parker was made), proof pressure was 16,000 psi. Thus it would seem you have a reasonable cushion. However, as recommended above, I'd have someone who knows his stuff check it out. Your chambers have been lengthened 5/8", which is quite a bit.