Agreed that a road from Lethem to GT is a good idea, integration into the rest of South America I'd disagree, Brazil has flirted for decades now with becoming an economic power house but for one reason or another it just hasn't happened (wider conversation) and Venezuela is a mess.
Manaus has close to twice the GDP of Guyana and is relatively close to Guyana but in real terms it might as well be on Mars, Boa Vista as nice as it is offers , well not very much, I hope it does help the smaller traders especially in agriculture but from a wider South American economic stand point I just don't see it benefitting many on either side but interesting non the less