Picking a cam and torque converter

I'm building a 390 (4" stoker kit) from my 318 LA engine to power my A108 van (adding aluminum heads and probably EFI). The purpose of this van will be to cruise on the highway but I want good low-end torque. I'm planning to keep the compression ratio low to run off pump gas. I'd like to retain my stock 727 transmission.

My question is what information do I need to gather and then how do I select the best torque converter and matching cam? The sales rep from Summit asked me about my rear diff, wheel size, cruising RPM, etc. How is all that information used to select a cam and torque convert?

I'd love to see some equations!

He also wanted to know if I was planning to "upgrade" to a four-speed transmission, which sounded great but also another project that I'm not sure I need to tackle right now...

Thanks