I would check for a leaking power valve.. I had this happen the other week.. they were fairly new valves, but all of a sudden i was having all sorts of issues.. pulled the carbs, broke them down and found the secondary carb had a wet power valve chamber.. I tested the valve and it held on a vacuum test, but i replaced it and we were all set again.running great..

I ran a Tri Power in my 4 spd 390 Mustang for 10+ years and it was a lot of fun.. I have since put it back to stock looking as i wanted to use the setup on a Galaxie.. But it still did all the right things when you dropped the clutch 

Hi, what thread is the coolant drain plug in the block.. It would have to be a tapered thread.. Thanks.

Thanks for the comments.. I'm not looking at going big on HP, around 450. Edelbrock heads, FPA headers and a cam, which i haven't picked yet.. so this is some good news that the block is ok to use. Its not a matching numbers block, just a replacement block that i had been saving for a rebuild, so to get the news that it had bad core shift and that the recommendation was to look for another block, was a bit of a let down..
I will take the advice and look at some custom pistons

I always thought the small hole was the first bolt that was installed, which then centered the remaining holes..