OK, I got the kit parts in today so I guess I'll update this thread with progress and see if it ends up working out.

I've used RG-174U for 100 W radios with no issues. Many manufacturers use it for internal radio connections. I prefer the coax with teflon insulator.
It probably would not hurt to add a #31 mix ring on both the input and the output of the amp. I'd skip the SMA connectors and solder the coax directly to the board.
BTW, I've been looking for a broken car audio amp to use the enclosure and heatsink from it for the HF amplifier.

Back in my homebrew days, I used simple silicon sealant instead of heatsink heat transfer paste. It worked just fine. Special paste was not available and we had to improvise.

Thanks for the reply. I planned to solder directly to the LPF board but since the amp has the neat little pads, and I've never soldered the edge mount SMA connectors, I wanted to just for kicks.

Has your amp arrived yet? 

The 174 will work. However, 316 is really the right stuff to use. Since you'll only need a few feet, spend a couple of bucks on it, it's cheap insurance. You've probably got to buy the connectors anyway, so just source a few feet of 316 from the same place.

Yeah, I will apparently need to order some TO220 mounting kits and I'll need to order the heat sink compound, plus a cooling fan and some connectors for the fan and PTT sockets.

I did find a youtube video from someone who built one of these, and a blog post that had a .jpeg of the schematic. 

https://www.youtube.com/watch?v=CeegQYZ25J0

http://pa-11019.blogspot.com/2016/11/diy-kits-70w-ssb-linear-hf-power.html

ETA it looks like you can get TO220 mounts with silicone insulators instead of mica, and they don't require heat sink grease.

The amp has not arrived yet. I'll take my time building it. I'll probably add some simple, homebrew bandpass filters since I have most of the parts anyway.

BTW, RG174 is used on many mobile antenna mounts and I believe they are rated for at least 100 Watts. Also, the internal coax on my ALS-600 is RG174 with clear teflon insulation. Also, one of my portable antennas uses about 20 ft of RG-174 to make it light. I never noticed a difference between 174 and RG-8X on both TX and RX.
Either way you go, 174 or 316, get the one with clear teflon jacket. It's a lot easier to solder.

Why do I keep clicking on cool projects when I know I don't have the time or place to dig into them... :-/

Just whittle away a bit at a time. Keep everything (parts, tools, etc) in a shoe box or tote or something so you can easily take it out and pick up where you left off whenever you get an hour here or there to work on it.

In the video I posted above, the guy said it took him about 2 hours to build. Even if it takes almost twice that long, it could be completed in a week of half hour sessions, give or take.

The parts kit for the amp arrived today. It is a surprisingly compact little kit. The whole thing, when completed, will hardly be larger than a deck of playing cards (but bigger when the low pass filter and heat sink are installed). 

My plans to use an existing CB amp enclosure and heat sink are probably going to change. Since this is such a small kit, it would be a shame to bulk it up with a much larger enclosure. I'm going to shop around for a smaller enclosure with an integral heat sink that will fit the amp board, low pass filter, and a cooling fan but without too much extra space. 

I have some RG316 on order and I'm still waiting to order the TO-220 mounting kits because I haven't decided if I want to go with mica or silicone insulators. With Mica, I'd need to order some heat sink compound. But depending on the enclosure type I order, I might need to get some compound anyway to connect an external heat sink to the enclosure.