First, how do you know if you are 180 degrees out on timing and how do you fix it,second, how would you know that your timing chain has stretched or jumped timing. Change out the distributor and thought it was in right, but seems not, when I try and start motor, it backfires badly

180 out u would have fire up thru carb trying to start. Been there!

pull the DR front (#1) plug & crank it carefully in bursts at the starter relay till you feel pressure coming up on your thumb on the plug hole. Bump it a bit more at the SR or with a 1&1/4" socket/breaker bar turn it further CW till the timing marks are at 15 BTDC. Set the dist housing so the magnet is dead even with the tooth that places the rotor pointing at or close to the #1 plug wire. See if it will start & holler back. check that the plug wires are in order 1-8-4-3-6-5-7-2 CCW on a BB and CW on a SB. EDIT If it's off enough that you have to reclock it I would suggest keeping the wires in the OE location (where #1 is for standardization which is forward and slightly to the pass side) and reclocking the intergear wherever it needs to be to achieve that. MORE EDIT I reread & what was the issue that you pulled the dist for and did it run before you pulled it?

To check chain stretch.Set the damper on zero by hand turning the crank with a socket.Pull the cap and see how much the crank turns on the timing tab compared to the rotor moving.IIRC no more than 8 or 10 degrees .After that its chain time.As said you probally have the dist in 180 out.Pull it and turn it 180.Rocky

I for sure have fire coming up from the carb when I try and start it

Car was running fine, then it started to break up at higher rpms just sitting there running at idle and had a little backfiring going on, then found that the distributor hold clamp was very loose(like real loose), installed another new distributor , tried starting it and now I get flames shooting out of carb and oh course it won't start

Before pulling the dist I would (1) set the timing marks on 15 BTDC making sure I am on #1 compression and see which cap terminal the rotor is at then (2) if the rotor at that point is on #1 I would with a 1&1/4" socket/breaker bar move the crank back or forth slightly as needed till the tooth is dead even with the magnet (best is to be coming forward "clockwise" at that point which takes the slack out of the chain) & holler what timing you have on the dampener at that position. Holler with what you see. (3) if it is on #6 then no need to do (2), just set the marks to 15 (#1 compression)/flip rotor/dist 180 to get back on #1/line up the tooth to the magnet with rotor pointing to or close to #1. (& if it ain't real close note that vac adv will shift rotor phasing CW on a big block. See if it will fire. What was the problem with the other dist other than the clamp being loose? EDIT Phasing-CCW on a SB