Rpm staying same

[Hotrodbuick]

Got my car all together and ran it good today and noticed car will not go over 4300rpm, at about 4000 it makes a thud sound. I tried pulling shifter to one and went nice an easy an car will go over 4300 I ran it till about 5200 and it made little boost. I'm gonna throw my other boost gauge with vacuum to see what it stalls at but has anyone had anything similar to this? At first I was thinking valve springs but now I'm leaning towards converter? Car shifts fine normal driving

 

[Hotrodbuick]

Also occasionally it will just pop out of gear into neutral. But hasn't done that in a while. Still trying to get bugs worked out and what all is done to the car from previous owner as he died.