OK, after a good night's sleep, I'm not quite as grumpy as I was last night, so let us begin anew.
I googled the 14950-030 you posted and clicked on Skip White's ebay site. The following are some quotes from there.....
"These pistons are for use with 6.00" rods. The compression height is 1.140 -14cc DISH TOP WITH 2 VALVE RELIEFS. 4.155 BORE SIZE, RING GROOVE SIZE, 1/16 1/16 1/8 and will produce approx. 10.5:1 compression ratio on 64cc heads"
OK, so we have that info. Now, a little further down, this is written.....
"These pistons have 2 valve reliefs, and are considered a true REVERSE DOME top -15.2cc."
Hmmm, maybe it depends on who's doing the measuring, I don't know.
"Premium pump gas with an octane of 91-93 puts you on the thin edge of detonation."
Yeah, I'd agree with that. All it would take is a short cam like you plan to use to make 'er rattle.
Now, here Skip is saying 10.5:1, but just for the sake of argument, we'll use your 10.3:1 SCR to figure dynamic compression ratio. Dynamic compression ratio (DCR) takes into account the static compression ratio, the stroke, the rod length and the closing point of the intake valve. Compression cannot begin in the cylinder until the intake valve closes, so that is what determines the cylinder pressure, along with the static compression ratio. After many years of doing this stuff, I have found that the Keith Black calculator works well for me and I will generally use from 8.0:1 up to 8.50:1 dynamic compression ratio for use with pump gas. Some fellows will build a pump gas motor up to 9.0:1 DCR, but I prefer to stay on the conservative side of things. Of course, using a tight squish of 0.035" to 0.045" will go a long ways towards preventing detonation also.
So, anyway, using a 10.3:1 SCR, 6" rod length, 3.750" stroke and an intake closing point of 35.5 degrees ABDC @0.050" tappet lift for the 12-600-8 cam that you want to use, the calculator spits out a 9.05:1 dynamic compression ratio. Now, if we use the 10.5:1 figure that Skip published for these pistons, we would get a 9.22:1 DCR.
My suggestion would be to use more cam with your combination, one with an extended intake closing point to reduce cylinder pressure and the tendency to detonate. Staying with the Thumpr series that you like, the Comp 12-602-8....
12-602-8 - Thumpr
closes the intake valve at 43.5 degrees ABDC and would reduce the DCR to 8.78:1 if you use 10.5:1 in the calculator, or 8.62:1 if you use 10.3:1 in the calculator.
Oh, and by the way, please stop comparing your friend's 350 to your 400. You have an additional 50 cubic inches and that changes the combination. You can't compare apples to oranges.