292 wont start after exhaust manifold replacement

[FL71C10]

I took my truck to a mechanic to have the exhaust manifold replaced. It's a 68 c10 with a 292 and 4 speed. No PS or PB. I bought a used manifold on this forum, and brought it to them with gaskets and new studs installed. these guys have been good to me and are honest. The one owner is an old drag racer who still races an old barracuda.

They decided to split the manifolds so as not to disturb the carburetor, and no bolts broke off during disassembly (which was my main fear, and the reason I took it to them in the first place).

I got there friday after work and they had the carb apart apart on the bench checking to see if a float or seat got messed up, as the truck would not start after they got it back together. It was popping and backfiring through the carb.

Then they thought maybe it had jumped time. They took out the #1 spark plug and moved it to TDC. They said the distributor rotor was off by like 40 degrees. They rotated the distributor to line it up just to see if it would start. It didn't seem to make a difference.

I did notice that it backfired and seemed to spit flames behind the carb between the carb and the block. I am wondering if maybe there is a big vacuum leak somewhere.

These engines have timing gears, not chains, right? If so, it wouldn't be very likely to jump time just starting up, would it?

I know they are not supposed to know what kind of timing gear every old motor has, but I was a little surprised they were so perplexed by this problem.

They are talking about pulling the valve cover this morning to see where the valves are at TDC. I'm going to make sure they know that there is no timing chain to stretch.

I am hoping the light of a new day will yield an easy fix this morning. I'm sure they were tired at the end of a long day and a long week on Friday.

Any suggestions would be welcome (except "drop in a new crate engine" or "go LS"?