Hello Tushar ji,
The GST rate applicable for body building of truck etc.. is 18%.
This has been clarified by the Department of Revenue, Government of India in their circular no. 52/26/2018 dated 09.08.2018.
The relevant portion of circular is:
12.1 Applicable GST rate for bus body building activity:
Representations have been received seeking clarifications on GST rates on the activity of bus body building. The doubts have arisen on account of the fact that while GST applicable on job work services is 18%, the supply of motor vehicles attracts GST @ 28%.
12.2 Buses [motor vehicles for the transport of ten or more persons, including the driver] fall under headings 8702 and attract 28% GST. Further, chassis fitted with engines  and whole bodies (including cabs) for buses  also attract 28% GST. In this context, it is mentioned that the services of bus body fabrication on job work basis attracts 18% GST on such service. Thus, fabrication of buses may involve the following two situations:
a) Bus body builder builds a bus, working on the chassis owned by him and supplies the built-up bus to the customer, and charges the customer for the value of the bus.
b) Bus body builder builds body on chassis provided by the principal for body building, and charges fabrication charges (including certain material that was consumed during the process of job-work).
12.3 In the above context, it is hereby clarified that in case as mentioned at Para 12.2(a) above, the supply made is that of bus, and accordingly supply would attract GST @28%. In the case as mentioned at Para 12.2(b) above, fabrication of body on chassis provided by the principal (not on account of body builder), the supply would merit classification as service, and 18% GST as applicable will be charged accordingly.
I have attached the above said circular to this Answer. You will find your answer on page 5 & 6 of the circular. I have highlighted the your answer there.
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