ok so i have a problem....transmission i think
 

well ill give a little back story i have an 86 gmc that i just recently put a ls1/4l60 in. well me being without absolutely anywhere to do it i got the parts and had a shop do it. any way after i got the exhaust done i drove it to my brothers house and on the way there it seemed like it took a little longer than normal for the throttle to kick in. like i was gassin it and the motor was revving and it just took longer than normal to get rolling. once rolling it went through first and second gear fine up until about 35 miles per hour and it just wouldnt go any faster. im looking for suggestions on possible problems, anything would be helpful.

oh and whether yall no it or not yall helped alot in getting ready for the install so thank you.