Need help with tranny

ok, i have a 79 Trans am with 403 olds, turbo 350 transmission. It was running bad for a while and poping so i changed fuel filter, ran the gas almost out cause i was pretty sure it was bad and put premium with octane boost/fuel cleaner in there. Help some and finnaly i got a timing gun and it was 11 degrees off and now its working pretty nice, big change, lol. The only problem is which i dont know whats really going on is when i floor the car it sometimes it bogs down and wont spin the tires and sometimes it does it really well, im kinda thinking its still my timing and gas but when i hit around 2000 rpms it always goes reallly good but i switches gears at around 3500-4000 rpms (when im flooring it), i know it redlines at like 5000 but usually it doesnt barely gets to 3500, if i put it in Low and take off it does the same thing at the start and after about 4000 rpms it will takes a while to get to 5000 or so. I aways thought if you were flooring it then it would switch around redline? I changed the transmission filter the other day and it shifts alot nicer but it still is shifting at 3500 rpms. If knows if this is a problem or maybe my engine is just old and cant doe those high rmps anymore (26 years old with 218k and i dont have any documents that shows it ever being rebuild but the tranny was around 80-100k ago), idk. thanks for any help.

thats kinda how my firebird is. when i floor it, it doesnt usually spin the tires and will seem to take awhile to get to higher RPMs. but when it gets around 2500-3000rpms, it'll take off pretty fast. anybody know how i can get it to spin the tires and take off fast right when i floor it?

btw, while i floor it, mine will shift around redline. so no problem there.

yea im pretty sure either the timeing or gas was my problem i can do a pretty nice burnout now, but it still shifts at 3500rpms, i shifted it into low again today and it got up to 5000 rpms pretty nice and sounded good too i wish it would do it on its own so it would shift faster plus when i changed the tranmission filter i must have messed up that linkage cause its really hard to move my shifter now but im sure i can fix that pretty easy. Im going to change my plugs now cause a few months 11 degrees off time might have done alittle damage.