The 351 in a 91 was only 240hp, the GT40 heads became standard as the 285hp carb'd HO engine in 93 IIRC they were not an option before then. Unless the previous owner upgraded the cylinder heads he's got the wrong specs. Common mistake, and the powerslot should nearly make up for the difference. If you visit the owner and start talking about these kind of details he may open up about other issues once he believes you're an MC expert.
As for the hesitation, it may need a carb rebuild - not too pricey.
Here's some info from Skidim.com:
351 ford high output (h.o.) cylinder head
indicated by 3 raised ribs on the end of the head.
2 raised ribs indicates a standard 351, 250HP [edit: they say 250, but MC's Indmar was 240hp] with hardened exhaust seats.
A flat head, with no raised ribs, indicates a older style without hardened seats, requiring Valve Tech or a lead substitute.
Flat Head and 2-rib heads are compatible, just make sure both take 14mm plugs.
(Message edited by HBSkier on June 27, 2006)