Engine doctor needed!!! Help 😆

[kcstreetracer]

Ok guys I have a 68 I've been restoring for a yr. Finally to a point of starting it. Here's what its doing. Go easy im not a real great troubleshooter.

73 350
New top end. HEI, Cam, lifters,rockers, pushrods, holley mechanical secondary carb( supposedly rebuilt ready to go)

Got everything buttoned up last night amd fired it up. Started for a second and died. Let go till this morning as it was late amd didn't want to disturb the neighbors. Today it won't start and once in awhile sounds like it'll start but won't. Periodic backfire no flames just a puff of smoke

Last night I checked valves. It has spark. 6.5 psi fuel pressure. Pretty sure timing is right as it fired last night. Has spark but won't fire with starting fluid now Today

Its a new flat tappet cam and I don't want to wipe the lobes out trying to get it going. Any direction for a newbie is greatly appreciated

Also is a new factory wiring harness and using the hei conversion wire repinned in harness.