Yeah, a 1515 would be a better choice. For 6s and 40mph go with something around 1600kv. I use the 1521 1y in my muggy and lst2, both are heavier than a truggy, and run much heavier wheels/tires. I have seen lsts run the cc 2200 motor (a 1515) on 5-6s and it has plenty of power as well. So a 1515 should suit you fine, and not overpower your truggy.
I think a 1521 in a truggy would be undergeared at only 40mph, unless a lower kv like 1200 were chosen. Otherwise, the other suggestions would do just fine
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz