5.0 a/c 134 conversion in 1970 stang - high pressure valve popping

I did it! now I have a 1993 5.0 FI comlete engine with its serpentine belt and compressor and all 1993 accessories with custom made a/c hoses, new condensor, new dryer, original a/c & heater box inside car (does not leak) in my 1970 factory air car. I have had the a/c system vacuumed and leak tested and re-charged - the a/c will cool super but soon it pops the high pressure relief valve. Any thoughts on how to stop this. The 134 freon amount has been adjusted to the proper amount vs r12 and now I have purchased from classic auto aire a new expansion valve but have not gotten it back together yet - I don't know if this will solve the problem but I hope so - I'm just asking in case this does not work. Somebody said something about an orifice tube system on later model cars if this does not cure it - what is that?

Getting the lines is the easy part - just take your old lines and old fittings and what they are supposed to connect to (fittings that the lines screw on to) measure how long you want them and go talk to any hydrolic line shop that custom makes high pressure lines - they will make them right up for you for less than 50.00 a line or so - maybe as little as 25.00 a line - you can find a company that does this by contacting a tractor repair shop, looking in the yellow pages, talking to an a/c repair shop, etc... this is very common... all they need is the specs you want and an hour later or so you walk away happy.